Venice
Like No Other Place
See Venice and Die. The two things you must do in life, go ahead and knock it out early for your kids. There are a lot of places that call themselves "The Venice of _____" -- or, compare the number of bridges to Venice. Listen. It's not even close. Venice is spectacular.
City of Water
As a kid, I would look out the window and imagine how cool it would be the asphalt street outside my window was water. Well, that's what Venice is. Your kids will be impressed. And, kids love touristy stuff, like souvenit baubles and gelato. Indulge them.
But Prepare to Spend
Splurge on a boat ride or three. You can do the cheaper ferries, the swift but expensive water taxis, or go all in on a gondola. Yeah, it's not "authentic", but I'm not sure there is any authentic Venice left. So just go with it.
Connects With:
The best flights from the US fly into Milan, so you can easily connect Venice to Switzerland and Tuscany. Or, fly into Rome and drive up.