We started taking ours when they were 2 & 4 and go every year, they are now 6 & 8. Didn't notice if you have boys or girls, mine are girls.
MK:
PHILARMAGIC - Fantastic!
Toontown for your little one - the characters are great and Mickey & Minnie's house are adorable.
Castle stage show is always great.
Daytime parade - Share A Dream Come True
Wishes - night-time fireworks.
Spectromagic - Night-time parade
EPCOT
Soarin
Turtle Talk with Crush
Imagination Pavillion is pretty cool
MGM
Indiana Jones
Adventures of the Little Mermaid
Playhouse Disney
Beauty & the Beast Live on Stage
The Great Movie Ride
Stars & Motor Cars Parade
Fantasmic!!
We don't do as much at Animal Kingdom. Make sure when you walk in, you pick up a park map & show guide. Especially at MGM, as you will have plan your day to make all the shows. As a general rule, we take every opportunity to visit characters, because they are so nice to the kids, we never miss a parade or a fireworks show!! As far as your DH wanting to "just hit the gate and walk around", it's not a bad idea at the Magic Kingdom. You don't have to plan your day around shows there, and walking around taking in the sites, is well worth it, as there are many hidden treasures!! The confectionary on Main Street is wonderful, the Barber shop does great "do's" for the kids for $5, Story Time with Belle is in a hidden cove across from Cosmic Ray Cafe, and you would miss it if you didn't know it was there. There are hidden Mickey's all over the park, that are fun notice. Alladin & Jasmine sign autographs near the Magic Carpets every night at 8:00. around 8:25 they come over to the Carpets, and ride with some of the kids. In general, the MK is indeed the Happiest Place on Earth, and we have the most fun, when we relax and take it all in!!
Everything everyone is telling you is daunting, so here is a link to the Park maps to get oriented before you go! Enjoy!
http://www.wdwinfo.com/maps/index.htm