Please help with an itinerary for 14 days. Desperate for all help!

Are you talking about this summer? I am afraid it could be slim pickings with accommodations. Italy's top sites and coastline are always crowded in summer, but it does seem to be a banner year for tourism, and you can see from the forum that the Amalfi area is getting a lot of attention.
A city-focused trip in December would be lovely. I would consider giving dad a break and doing a beach trip this summer, and plan for Italy in winter -- then perhaps try the Amalfi Coast next year. A bonus is if you go in winter, Pompeii will be delightful instead of an inferno.
Remember, three nights is only two full days--it's the bare minimum for any of these cities. Also note that the Amalfi Coast is far from the only dreamy coastal part of Italy. You sounds concerned about crowds, which is something my family also does not enjoy. There are many other parts of Italy that you could enjoy without the crowds.

I'm going to jump in with a strong vote for December and January, although it means no beach time. Rome, Florence, Venice and Pompeii/Herculaneum are delightful at that time. My one trip to Pompeii was the first week of December. We stayed in Pompei town itself for four nights and visited Salerno for the Christmas lights, Pompeii, Herculaneum and Naples for the Archeological Museum. It was relaxing to do one thing per day with no crowds. The weather was great - about 68 F / 20 C and sunny. No guarantees on that though! It will be comfortable but not beach weather anywhere in Italy. If you could see your way to doing this instead of Amalfi and beach time, I think you'd meet your objectives of taking it easy AND seeing important sights. Then do the Rome/Florence/Venice circuit, which would also be more enjoyable with smaller crowds.

In cities at Christmas time, there are still opportunities to eat outside (often with heat lamps) and be outdoors without hardship. And it makes the museums easier to take when you're not dripping sweat squeezed in shoulder to shoulder (or to armpit if you're short like me).

Note that there are Christmas crowds - the holiday season doesn't end here until Epiphany on Jan. 6 and of course European school kids are also on vacation. But it's nothing like summer. July and August are just as unbearable as you're picturing. I strongly recommend you take your summer beach time in the States. It's just gonzo here already this year.

Well, that's my two cents, worth what you paid for it.

Hello! Yes. We were talking about doing this in the next few months. If we reserved the Amalfi Coast for another trip is there perhaps another Italian Itinerary you might suggest in it's place? Something more realistic, but also without the crazy crowds. (Given we would probably still want to hit Rome and Venice). Or am I being way too ambitious considering the timeline and also our desire to avoid crazy long lines and crowds?

If you want beach, without as many crowds, look at the Puglia area. It's not nearly as popular but has a lot of beach options. I've never been there but it's popular with families.

I'd personally fly into Venice, do Venice, Rome, then end at a beach area not in Amalfi. If you can fly out of Bari you'd have lots of options in that area to look for beaches. With kids your age, and a husband not interested in site seeing, I'd skip Florence personally.

Another option is Rome, Tuscany/Florence, then a beach up in that area. You could stay near the Cinque Terre, but not in it for less crowds, maybe La Spezia.

Hello! Yes. We were talking about doing this in the next few months.
If we reserved the Amalfi Coast for another trip is there perhaps
another Italian Itinerary you might suggest in it's place? Something
more realistic, but also without the crazy crowds. (Given we would
probably still want to hit Rome and Venice). Or am I being way too
ambitious considering the timeline and also our desire to avoid crazy
long lines and crowds?

Even as someone who has been to Italy 8 times, I would have a hard time planning a last minute trip to the "big three and then some" during high season for four people with high expectations. Many hotels and tours are likely already booked up. I am not saying it cannot be done, but it will take a lot of work. I have really enjoyed the coasts of Marche and southern Tuscany, but anywhere on the coast is going to book up far in advance and be crowded (maybe with Italians and not everybody but still), and I am not familiar enough with summer travel to advise.
You could devote an afternoon to looking at hotels and determine what kinds of things you could get tickets to (or not) and then decide. If I was going in summer with Rome and Venice as musts, I would put a mountain trip in the middle. good luck!

valadelphia and Nelly have, in my opinion, a very good idea. Do dad's laid back beach / pool thing here in the states this summer. Then do Italy (Rome and Venice) during the Christmas break. While there will still be some crowds, the heat will be way less and lead to a more enjoyable trip. Just offering food for thought.

If you want to go this summer, you could consider skipping the beach and instead look at staying at an agroturismo or masseria in Tuscany (with a pool). This would be really relaxing, great food, beautiful scenery, and you could probably day trip from wherever you stayed to visit a town or two if some of you need more 'action'.

We just returned from Puglia and also recommend that area for beach with a bit less crowds. We took the train from Rome to Lecce (5.5 hours) or you could fly to Bari (or train to Bari which would be 5 hours). We loved Lecce and also Ontranto would be great to stay for several days for relaxation and sun and sand. AND history! Or we stayed at two great masserias in Puglia -- both were inland but with great pools and food and the beach is very nearby.

Also how does one travel with so much luggage if we have to do trains?

The somewhat snarkly unhelpful answer to that question is don't take so much luggage. But seriously our family of four just did a 2 week trip with carry-on size luggage (travel backpacks + daypacks for each of us) and I don't think any of us missed 'stuff'. We did do laundry a few times, but things will dry quickly in the summer, etc. Your kids are younger so the travel backpack might be a bad idea, but even if you just did one checked bag for the kid's 'stuff' and one checked bag for the adult 'stuff' that's still not a terrible amount to manage on a train. I'd suggest browsing the "Packing" forum for ideas on how and what to pack for a trip like this (I have my packing list in my recent trip report if you want to look there). The trains all have luggage racks both overhead and at the end of the cars for bigger bags. You do have to move/carry everything yourself, but if you pack judiciously it's really not that difficult.

These are great questions you are asking so keep asking!

I don't know how far in advance they book, but you may not be able to get advance tickets to the Sistine and Colosseum in July or August. Getting them on site may require standng in line for hours. Hopefully others on the forum will address this issue. Also, it's been reported that you will not be able to see the Pieta or Bernini altar in St. Peter's this summer.

I totally agree with Nelly and Valadelphia that winter would be a better time for sightseeing in Rome and Venice. but that leaves your husband’s wish for beach or pool time unfulfilled. If that is a must, and you really want to go this summer, I can suggest a possible itinerary that avoids the Amalfi coast/Sorrento crowd crunch (which wil be horribly crowded and probably all booked up anyway).

In between Rome and Venice, stop not in Florence but in Siena. Book 2 nights at the agriturismo Mulino Di Quercegrossa, which is just 6 miles out of town. You should be able to take a taxi there from the train station. They have 6 pools of various types; one with a current that ends in a pool with a waterfall. Our teens loved it there. Just don’t go on a weekend. On your departure day, leave early, hopefully find a luggage storage at the Siena train station, and take a few hours to see beautiful Siena. Then catch the train to Venice )you will change in Florence on the way).

https://www.mulinodiquercegrossa.it/en/index.html

Look it up on booking.com to see more photos and maybe book there:

https://www.booking.com/hotel/it/mulino-di-quercegrossa.html?aid

In Venice, you could if you wish, book a hotel on the beach at the Lido, instead of in the city itself. You and the kids could ride the vaporetto back to the main island each day for your sightseeing, and your husband can stay and relax by the beach. Give the kids a day there too; maybe rent bikes and ride around.

It is not ideal, but at least it gives everyone something to make them happy.