We need more info about your plans.
We get a lot of Disney people trying out Universal for the first time. That's great, but most of them drive up only taking one precious day away from Disney.
For kids that young, either park has kiddie rides, play grounds, and shows the kids will enjoy.
I'll vote for USF because they have Fievel's playground, Barney show, pet show, Shrek, Jimmy Neutron, and the characters hang out at the front of the park. Bring a change of cloths for the kids, they'll get wet in the play ground.
Arrive early at 08:00am. Lines are shorter for the attractions. The kids will become cranky in the afternoon, and it's best to leave because they'll just zonk out.