You will be fine! Some tips:
Use the days without EP to do rides with lower wait times. Also, Hagrid’s, Velocicoaster, and Pteranodon Flyers don’t take express anyway, so do those on your non-EP days, too.
Download the app so you can mobile order food and monitor ride wait times. Since you’re used to WDW, you’re probably used to FP+, but EP is not quite the same in that your wait will likely be about half the posted wait, so could still be significant.
Make sure to check the app for show times, because the entertainment at UO is very well done.
Getting ADRs at WDW is practically a contact sport, but UO is much more laid back. Still, you might want to book a few reservations in advance because the lines for counter service can be long at peak hours and it’s no fun having hangry kids. The app has a list of restaurants along with menus, so take a look to see what appeals. I don’t know how old your kids are, but most places don’t list their kids menus. Nearly every place has something (even if it’s only a few choices), but you usually have to ask.
If you are not going to HHN, you will be cleared out of US at 5. You can still go to IOA, but be aware that a lot of other people will be doing the same, so expect longer lines, especially if you’re planning to take the Hogwarts Express to switch parks.
Breathe and relax - you’re going to have a great time! There’s not really much planning like a WDW trip.