I'm not sure of the details of your trip and if you're staying on WDW property or not, but my first suggestion would be to go to allearsnet.com and/or the official WDW, SeaWorld and Universal sites to see what really appeals to you in those separate parks.
While I don't know your children's or your specific interests, I have 4 kids ages 6, 5, 3 1/2 and 2 1/2. This is what I would probably suggest:
2-day MYW w/hop option tix for WDW, using them for:
DAY 1: Universal (or IOA) whichever you decide. My kids have never been, but from what I hear, their faves would definitely be Spiderman and superheroes, along with Dr. Seuss land and Shrek 4-D.
Day 2: Sea World (with a character dinner at Chef Mickey's to kick off the Disney part of the trip)
DAY 3: 1/2 day MK, 1/2 day MGM
DAY 4: 1/2 day MK, 1/2 day AK
I'd skip Epcot for now. I'm not saying it isn't worth it, because I do enjoy the ambiance there -- maybe even go there one evening with the kids in a stroller and just walk around... if you're trying to fit in a bit of everything, this is the one I'd leave out though for your kids.
You could also choose to go full day MK and split the 4th day only, btw. MGM and AK. But I found that there's something about hanging around MK a bit everyday if possible, is such a magical thing for the kids. Even though it's all Disney... they really love to be at MK most... this way they get a dose of it for two days instead of just one and done.
I also like the idea, of whatever you do end up doing, do the trip in chunks, meaning the days you spend at WDW, do them consecutively to provide more of an immersed Disney experience.
By the way, this is pretty much how I would rate the parks with kids in this age range: (JMHO)
MK
MGM
Universal/IOA
Sea World
AK
Epcot
Hope you have a blast whatever you decide to do!