Barcelona to Rome and back

You are really asking for a trip planner and that is hard for us to do. My first recommendations is to consider an open jaw or multi-city ticket. Into Barcelona and home from Rome or reverse. You waste a lot of time and money back tracking. With 16 days you are somewhat limited. Every time you change location you lose at least a half day if not more. Since a lot of your comments about locations to visit are in Italy and the Aips. I might consider flying into Switzerland and head south to Rome via train. Or even into Milan. Barcelona to Rome is a long way via land and to do a circle does not make much sense. And for your fist trip avoid the car. It is a hassle you don't want especially when it comes to parking and the dreaded no travel zones.

You need to spend sometime with guidebooks, a map of Europe so that you understand distances involved and especially travel videos to give you a sense of what is important to you. Your local library is a good place to start.

There are still travel agents sounds like you may need one. Are you planning on renting a car & crossing borders? People on this forum can not plan all your details for you. It works best if you state a plan and ask for insight. What you will save in airfare you will pay in rental cars. You do not need or want a car in Rome.
Good luck in your planning.

Please consider open jaw first of all. I bet in the end you will save the difference. We do it that way all the time. We have traveled with family…adult kids, grandkids. Try not to be on the move every day. Kids need to play at playgrounds. Laundry will need done. Research carefully your stops. Trains are a great way to travel. We dont rent cars in Europe…but many on this forum do. There are good websites to help with train travel. March will give you chance of rain or nasty weather…esp. Alpine regions. You have alot to decide in 6 mnths! Then reservations for flights, lodging and often tours/museums need reservations well ahead. What do those you will travel with suggest or prefer? Find a travel agent. Rick Steve’s guide books and videos at library. Seek out those you know that have traveled these places. Its hard to give more advice until you can get some more solid decisions made. Its a lot to plan and your time is getting limited if you go in March of 2026! Not trying to discourage…but being realistic as a traveler. Best wishes!

I think you will probably eat up most of your flight savings with your transportaton back and forth from Rome. I would very much recommend thqt you decide where. you want your trip and try to find reasonable flights close to those locations. Particularly if you are travelling with young children!
It sounds like most of your interests are in Italy. There are many international airports there, Milan, Florence Venice, Rome and Naples. There must be some flights similarly priced. With 16 days and young children I would focus on one country or region, either Spain with Barcelona flights or Italy. It is too early to train through the swiss alps and I'm not sure where you will drop your car there anyways.

Yes, this is a wishful plan, just like your name, but that does not mean it is practical or wise.
First, early March is great for skiing and snow in Switzerland, but Cinque Terre is most popular in the Summer, and could be chilly when you visit, as will also be most of Spain and Italy.

No, you should not drive from country to country if you have never been there before, especially in Winter. We have driven in many countries all over Europe, and my husband is a skier, so he also drives in snowy locations all over the U.S. and Canada.
Switzerland has some of the best trains in the world--we never rent a car when we visit. There is absolutely no need for a car in the big cities like Rome and Barcelona, and you might not find parking, either.

IMO, each country requires at least a week to get to know it a little (Spain, France, Italy), so trying to do them all in two weeks becomes more of a rush-around then a meaningful and enjoyable experience. It sounds like you do not really even know what you want to see or why. Traveling with children adds a different dimension. In order to see the Sistine Chapel you have to walk through the Vatican Museum--this is not always tolerated well by young children.

Yes, go to a library and do some research--but I know many people are often too busy to meaningfully plan a vacation in detail as I do.
You cannot go to a travel agent yet, because you do not know yet what you want or where to go. Also, they need to make a profit on commissions, so they usually prefer to book hotels and tours and cruises first. Finally, learning about local public transportation is part of the fun and the adventure of a trip.

So, here are some great more expensive tour companies you can just look at and get some ideas for itineraries and locations (Yes, I like RS, but I think he appeals a bit more to budget travelers):

https://www.trafalgar.com/en-us/search?text=Switzerland&brands=Trafalgar&useEmbeddedCards=true

https://www.tauck.com/destinations/europe-tours#itineraries

https://www.adventuresbydisney.com/destinations/europe/

Have fun planning, and good luck!

a couple young kids

What is 'young'?

Have you tallied up all the transportation costs and have a real savings by flying in/out Barcelona?

Trying to do Barcelona, the Alps, Cinque Terre and Rome in '16 days' ( how many nights in Europe?) and get back to Barcelona for a flight home seems quite ambitious.

How many people total? Depending on the number, the feasibility of a car rental may be affected.

When the time comes to buy train tickets, you should buy them from the companies operating the trains, but the Deutsche Bahn website is very handy for checking schedules during the planning phase. It will usually not give you fares for trains outside Germany. I don't think you'll find schedules for March yet, but train schedules don't change all that much from month to month unless there's some sort of maintenance issue. Just pick dates in November to get an idea of travel times.

https://int.bahn.de/en

If you click on "Details" for a departure time, you'll see a listing of all the trains and transfers along the way. On that screen you can click on "Journey Information" for one of the travel segments to find out all the stops that train makes along the way. All you need to begin your research are your starting and ending points.

Nothing you mentioned in your first post sounds like a car would be helpful; quite the contrary. There isn't really an obvious loop trip when you travel Barcelona-Rome-Barcelona. Using a car one-way across borders would probably result in an extremely high drop fee; it could be as much as 1000 euros. You can go to ViaMichelin.com and take a look at the suggested routing. ViaMichelin estimates 84 euros in tolls and 126 euros for fuel (in a Clio V, which I assume is a small car) for the one-way trip on a southerly route that doesn't take you close to Switzerland. Then there's the expense of parking.

To explore the cost of a linear, rather than loop, trip, price out multi-city airline tickets for flights into one airport and out of another. I use Google Flights for my transatlantic trips.

I love Barcelona, but it doesn't pair well with Italy unless you plan to fly between the two. Personally, I hate flying in the middle of a trip. It takes a lot of time due to the need to show up so early at the airport and the time required to get from hotel to airport and vice versa. You also have to be aware of all the extra charges you can run into on budget airlines--luggage fees, seat-selection fees, etc. I think adding a separate flight from Barcelona to Italy would eat up a lot of the potential savings from flying into BCN.

I suggest doing some research on March weather in Switzerland. It's not necessarily a good time to travel there except for skiing. And everything about Switzerland is expensive.