If you want to do everything-including the spells, Ollivanders, all of the shows (five total- there’s two diff plays in Diagon), all of the shops, taking in the details, looking in the restuarants even if you don’t eat there, just taking in all of the tiny details- you need two days if you want to do anything outside the wizarding world. If you’re just interested in the rides, catching some shows, and quickly browsing the shops, one day is fine. You need a park to park ticket to ride the Hogwarts Express.
There’s not much kids can’t do, though they may not be interested in everything. The big coasters may be the only things younger school age kids are too short for. You can find height restrictions on the app and website.