Hi all, taking the family of 4 (kids are 8 and 13) next summer on the british airways 500 dollar tickets was able to book. here are facts for my trip

-flying in and out of London from the USA
-cities we must visit- London, paris, venice, sienna

proposed itinerary

London (3 nights)
train to brussels- 1 night
train to bruges- 1 night
train to paris 4 nights
overnight train to venice (3 nights)- train just for experience vs flight?

from there I'm not sure on a couple options, we have 8 nights left to visit sienna and one more city probably. (we can also add a night to one of the above cities)

Here are my questions

1. best way to get to sienna from venice? should I stay 2 nights in Florence and do a day trip to sienna? or possibly stay one night in Florence and and direct to sienna?

2. and then last 5-6 nights of the trip deciding whether to go to rome (beautiful but we have been there before, or fly to Barcelona (wife really wants to go). leaning towards Barcelona but a shame to skip rome since we are so close. or do a quick stop in Rome and then Barcelona?

3. best way to get back to the united states? we have a 4pm flight the day we depart, should we just do a flight back to London that same day and then fly to the US? Do we have to fly the same airline to get bags transferred? Or should we consider flying in the day before and staying one night in London and then flying to the USA?

trip is kind of a whirlwind, which we normally don't like, but its the kids first time to Europe.

I'm also debating on our inbound flight taking a connecting flight straight to paris and staring our vacation there, so the leg home(when we are more tired) is easier.

Belgium visiting quickly for some beer producers I want to visit. cantillon,drie fonteinen and Westvleteren.

we've been to all the cities before except Belgium and Barcelona.

Any other cities between paris and rome that are really compelling? that would allow us to skip Barcelona?

For every major city, we plan to do the guided full day whirlwind tour one day of most of the sites (with line passes) and then explore leisurely the rest of time.

Any portions of our trip where we can and should rent a car?

Thanks for any feedback and reccomendations to our trip.