I posted on your other thread, but this looks fantastic to me. I've been to all those places and I think it's the "right" amount of time in each place, especially with those ages. (I took my daughters, 13 and 16 and/or my 18 year old son to the places on your list.)
I think the earlier you are in Italy the better as it will continue to get more crowded and London is more organized and easy to get around in large crowds in my opinion.
I don't think you should switch Amsterdam and Paris. I think you should consider taking the Eurostar from Amsterdam to Paris, then Paris to London.
I would not spend 12-18 hours and multiple station changes in multiple countries taking the train from Venice to Amsterdam. If you book now and don't have a lot of luggage, flying will cost less anyhow.
And yes, I think you have enough time for a day trip in each of those places, but there is also enough to do in each of those places to keep busy without it. (Paris is much, much smaller than Rome and half the size of Amsterdam, so that would be the first place I'd think of a day trip.)
I'd try for a hotel in London and consider an airbnb in Rome. Paris, you could go either way, but I don't find the Eiffel Tower/Arc de Triomphe areas are nice areas to stay overall... a pretty neighborhood with cafes will suit better and you won't fight the Eiffel tower crowds and street vendors every single day to see those things once.
Almost anywhere in Amsterdam is a cool place to stay, in my opinion.
In Venice, if budget is any concern at all, you'll want to stay in Mestre/Marghera and take the train or bus in. It will cut the cost in half or possibly more. Staycity Aparthotel has huge clean rooms for a very reasonable rate and there are coin laundry facilities, which you will need at that point in your trip. (There are very affordable laundrettes in Amsterdam that charge almost the same to wash and fold as it would cost you to spend hours waiting and doing it yourself, so I suggest you drop off on your way out one morning and pick up your clean folded laundry on your way back in the early evening.)
I have a suggestion that my young travelers enjoyed. In London and Rome in particular (due to size and number of sites), leave your last day loose and make a list of "that would be interesting" or "oh, I'd love to see that/go there!" as the week goes by... use the final day to see those things and/or return to your favorite gelateria/cafe/pizzeria. (This is not practical for major sites where tickets will sell out, such as the Coliseum, but I doubt you'd leave that til last minute anyhow.)
We also found that choosing restaurants ahead of time including casual dining/pizza (based on what attractions we'd be visiting) greatly reduced irritation as there was less time spent fiddling with phones, trying to get everyone to agree, etc... The trick (especially in italy) is to make sure you choose at least one backup nearby as lines can be long, places can be unexpectedly closed, etc. You'll want to make reservations wherever possible with a party of your size if you are planning to sit down or have particular places you want to eat. In Rome, you could wait hours or be turned away entirely at many places and the same could happen in the other cities depending on the restaurant.
This will be amazing... absolutely amazing.