Here's the link to the Italian 'Trenitalia' official Website, which should give you most of the answers you're looking for:
Trenitalia - Homepage English - Trenitalia
You can book Florence to Pisa and Pisa to Venice through this site.
Take the train to Pisa. It takes around an hour.
Going from Pisa to Venice you'd have to go back to Florence in any case to change trains. The journey from Florence to Venice is around two hours.
As you don't wish to feel rushed, calculate at least three hours between arriving at and then departing from Pisa station.
Personally, I wouldn't make the detour to Pisa, but if your kids really want to go I guess you have little choice
