AK can easily be done in half a day, but we routinely skip this park on our trips. Our kids are 7 and 4, and we've been going frequently since the oldest was 3. They honestly don't really enjoy AK. We go occasionally, just because Mommy likes Festival of the Lion King, and they enjoy Kali River Rapids, but then they are ready to go. The park is SOOOO hot! And we don't even go during the summer, so I can't imagine what it would be like then.
On the other hand, MGM is their 2nd favorite park - MK, of course, is first. This is their favorite place to see the characters, and they really do enjoy the rides. My oldest is a daredevil kid so she loves Tower of Terror, but even the youngest enjoys everything else at the park. Playhouse Disney has always been one of his favorite shows. Fantasmic is all of ours favorite light show. It's just one of those "can't miss" parks for us.
It really all depends on your kids. My kids don't really enjoy zoos. Yes, I agree, AK is way more than that. But it is still quite a bit of just looking at animals too, and my kids just don't go for that. If your children love watching animals, then definitely go to AK. If that's not their thing, I think they would enjoy MGM better.