If you are staying onsite, IMHO it is a real benefit to get to the EMH park early so you can be closer to the front of the "pack", especially with little ones. I expect you will want to hurry back to Fantasyland and Buzz Lightyear and those first few minutes can make a world of difference in terms of wait times for rides.
From experience: if you plan to ride Dumbo, do it first. Get in line with the kids while someone else in your party gets fastpasses for Peter Pan or Buzz. When they get back they can get in line behind you so that when the kids are done riding, if they want to do it again, you can pass the kids to them for a second ride with little or no wait (this is famously known as the Chuck Bubba Relay from the Unofficial Guide to WDW and it REALLY works!). After the first 30 mintues or so, the line for Dumbo is tends to get HUGE and will totally destroy any plans you have for other rides.