Itinerary assistance!!!

You have some long travel times. Venice to Munich is direct, I believe, but something like 7 hours. Normandy is easier with a car. How are you planning to get to Barcelona? I would really think about whether your sons can handle Dachau. I'm in my 50s, I can't handle Dachau. If you think Dachau might be problematic, I think there are other ways you sons can gain the same information. I enjoy Munich, you might just spend a day touring around the Marienplatz. If you live close to DC there is always the Holocaust museum that might be more gentle.

EDITED TO ADD: Is it possible to start your trip in Barcelona so you could fly directly there? And slide Paris and Normandy in after Berlin?

Have you traveled to Europe before with your kids? Do they pack and move quickly? Rule of thumb: X days in a city requires X+1 nights. You use about 1/2 day each time you move, some of your moves look to be all-day treks. How are you planning to get from Normandy to Barcelona? When are you going to do laundry? Shop for toiletries and incidentals? Does your cruise dock overnight in Venice? If not, you should spend at least one night there to see this unique city.

You are spending lots of time in transit, particularly with children and then short changing the time spent in most destinations - Paris and London each merit 5 full days there to just scratch the surface and you can easily fill 4 full days in Berlin.

Are you not going to spend time in Venice when you are there? Two full days (3 is better) can cover the essentials.

Excluding the cruise, you have 18 days to cover 7 locations in 5 countries and several places are going to involve at least half a day’s travel door to door eg France to Barcelona and Berlin to London. Venice to Munich is about 8 hours door to door. Excluding transit, you have about 14 days to cover 7 locations in 5 countries. Exhausting! Personally, I would be looking to reduce your number of destinations and plan out how long you are going to spend in each place and allow a sensible time for your many transfers.

I wouldn’t take a 10 year old to Dachau, but if a concentration camp is a must, then you could consider seeing Sachsenhausen from Berlin to reduce your number of destinations.

In Berlin, there are a number of good guided walks that will give you plenty of WW2 background. The museums are easy to cover yourselves - you don’t need a guide. Everything is in German and English.

Does your cruise start from Barcelona? If not, I would drop it as it’s a geographical outlier.

You're possibly trying to see too many places and travel too many miles once you get there. And I agree that London and Paris are more important than the time allowed. Munich, Venice and Berlin are also worthy of more time. Distances in Europe are much farther than many realize.
You might start in London for 4 days and skip Normandy altogether.
Fly to Barcelona on EasyJet.com inexpensively and stay until your cruise leaves.
Grab an extra day or two in Venice after you get off the ship.
Then head north on the train to Munich--a long day. Munich is a great city for youngsters with many sights to see including palaces and a great science museum. And don't hesitate to take the kids to Dachau as it's done very tastefully--and important.
I don't see where you have time to go up to Berlin as it again is worthy of 4-5 days to see properly and you just don't have the time.
Take the train over to Paris from Nurmburg and fly home from there.

Way too much travel.
Three days in Paris is not enough, neither is one day in Munich three days in Berlin and three days in London.

I have been to Dachau and not saying you should not go there, it is a learning experience that should be done. Not sure that I would take a 10 year old there. It is not as upsetting as Auschwitz in Poland. My Wife had nightmares for several days after we visited it.

Dachau was not an extermination camp like Auschwitz, it was a concentration camp.

Normandy is wonderful, loved the historic WWII beaches, American Cemetery and especially the Bayeux Tapestry detailing the story of the Norman conquest of England. It is 900 years old and looks absolutely new. Don't miss it.
Why aren't you staying at all in Venice?

Nuremberg is worth a visit, make sure you visit the site of the trials as well as the grounds of the rallies now turned into four sports fields.

Why not save London for another visit, and also take in some of Britain, there is lots to see other than London.

See more by seeing less.

Since you don't have airline tickets yet, I'd agree that I'd do London first, then Paris - doing at least 4-5 nights in both locations. Fly home from Germany so you don't have to double back to London at the end.

If you do this you'll want to get your Eurostar tickets from London to Paris now. They go on sale 6 months ahead so are already booking for August 9. You may have missed the least expensive fares but they will never be cheaper than right now.

Bootbuilder ----

Most of these people responding have lots more experience than we do - but just to throw our hat in the ring, a few years back while visiting the Cinque Terre , we tried to visit Genoa, Milan, Verona and the Cinque Terre in 15 days.

It seemed like plenty of time, hoping around by train --- but honestly we were exhausted by the time the trip was over and decided for the future that unless we had a full month to wander around, we would only pick two places, because these folks are absolutely correct in that every time you pack up, go to the train, and move.... you loose a day.

Good Luck

I suspect the OP’s cruise originates in Barcelona and ends in Venice; hence that location.

I don’t think your itinerary is un-doable as long as you know you have to make choices and that you can’t see everything. The cruise in the middle will provide some rest. I do agree with a PP that I would not take boys that age to a concentration camp though.

Have you been to Europe before and know what it’s like? If so, this itinerary may work for you. If not, just build in some “you’ll probably get lost a coulple times, the kids will probably melt down at least once and you’ll have to abandon plans, jet lag will get you at least one day” attitudes to be realistic.