At Epcot, your kids may be tall enough for Test Track. This was our 4 yr old son's fave ride! It is very fun, but it has some sharp turns, and near the end they spray green "road grime" stuff at you, and it almost looks like you will collide w/ some objects during the ride. However, we prepped our son, made a joke out of the road grime, seeing how loud we could yell "EWWWW! GROSS!!!!" He was laughing so hard from that, he hardly noticed much else.
Also, the maelstrom ride in Norway has a Troll and the boat gets turned around in a storm. Maybe take them on the boat in Mexico first (very slow and sedate). When you go to Norway, show them the wooden carved Troll in the store, and the toy trolls, and ask a cast member to tell the kids about the trolls. You may be able to get them on the boat!
Oh, and there is a storm in the Tiki Bird room in MK. Iago the Parrot upsets the Tiki gods, and sets off a storm, etc. It can get a little loud.
Come to think of it, if your kids are at all noise sensitive, pick up the foam earlplugs they sell at the pharmacies. They are inexpensive and relatively comfortable, and may save some parts of the trip. Music from the parades can even be a bit muh for sensitive ears.
PS if you an get it, there are a couple corny Disney sing along tapes that take place in WDW: Beach Party & Camp out. They give a good idea just how much bigger the characters are, compared to the kids. Especially Goofy! He's SO tall compared w/ the average 4 year old.
Good luck and have fun!