I agree Wilderness Explorers is just what your looking for. Its all over the park. They will love it.

Also, look into Sorcerers Mickey in the MK. You sign up in the fire house as soon as you enter MK. The kids get trading cards and you fight villians on hidden screens around each land. The 6 year old will get the concept but your 1 and 4 year old would probably enjoy it too. My kids are obsessed. I have a 3 prong folder with baseball card holders in it to keep the cards organized.

There is also something similar in Adventureland but you get pirate maps. I'm drawing a blank on the name. I'm sure someone on here can think of it.