Universal Studios has the London Diagon Alley section with the Gringots coaster. Islands of Adventure has the Hogwarts section with the Hagrids and Forbidden Journey rollercoasters. Both sections have plenty of shops and stuff to see and you can buy wands and have butterbeer. There's a HP train between the parks, but you have to have a park-hopper to ride it. You may consider two days for Universal.

I would say that your two youngest will not be able to do 1/3 or more of the rides at Universal.