Is money an issue, like it is for most of us?
Disneyland should be less expensive, depending on where you live and how much the airfare is, but you can still get close-in reasonably priced hotels at DL and it's easier to get to outside of park restaurants. For young kids, DL is fine. Most young children's first pick for parks is the orginal DL or Magic Kingdom at WDW. Fantasyland is custom made for them!
WDW, on the other hand, has the Animal Kingdom which is ideal for children of all ages. For young children, MGM and Epcot are only so-so -- of course that's a generality. MGM has a few kid oriented shows like the Little Mermaid and The Muppets, but it is better for pre-teens and teens. My kids think Epcot is a bit of a snooze except for the Test Track. This is why we started with Epcot, then MGM, then Animal Kingdom when we went. Of course, you can always go to WDW and only go to 2 of the parks.
Given a choice for a second day at one of the parks, our kids have always selected either DL or MK, hands down! So I think DL is a fine place to go and preferrable if cost is an issue.
At WDW, there are also a lot of other things to do. For kids 10 and up there is Disney Quest. And there are the water parks. For our money, we'd rather drive to a local water park. Our kids have no interest in Downtown Disney.
Happy planning!
Toodles
WDW 79, 83, 99
DL 91, 92, 93, 95