Hi Rebecca,
The week of July 4th may be a pretty busy week in the parks but you can still have a great time w/ a little planning. Because you are traveling during peak season, I would spend both days at MK. There are so many things to do there that your kids will love (you too). If you are staying on property, I'd take a mid-day break when the temperatures get hot & go back to your resort for some pool time or naptime. You can come back when your kids are refreshed & ready for more action.
Fantasyland & Toontown are super popular with the kiddos. I'd get there as soon as the park opens, go to Fantasyland, then Toontown & keep working your way around - avoid Fantasyland in the afternoon as it gets really bottlenecked with people. Decide what your "must do" attractions are & do those 1st. Take advantage of FPs to reduce your wait time. To learn more about FPs & which attractions offer them, click
here.
If you decide that you want to do a 2nd park on another day, your DS4 would probably really enjoy DHS. Lights, Motors, Action Extreme Stunt Show is always big fun for the little guys as well as Toy Story Midway Mania, Playhouse Disney (don't sit at the very front - you can't see well), Muppet Vision 3D, meeting Disney Pixar characters & the Honey, I Shrunk the Kids playground.
Before you go, check out the
Traveling With Little Ones section on the WDW website for height requirements & other helpful tips. Make sure you take stroller fans, water misters, stroller covers in case of rain, sunscreen, hats & most importantly keep your kids well hydrated.
Wishing you many magical memories!