I disagree with some of the PP's. My 5 yo son loved Epcot. His favorites were test track, Soarin, the kidcot stations, making his own take home robot for free, the soda taste testing, the maze in england. A lot for an energetic boy to do! We went twice with a 5 yo to Epcot in one week.
The only thing he liked at MGM was Star wars and the honey I shrunk the kids playground, so if you aren't into those I'd skip that one.
Have fun on your trip, it is a great age to go!
I am with Oldbay. There is a ton for kids at Epcot. My DD who is now 6 loves test track and Sorin, and did at 5 years old too (maybe later 4's when she was tall enough.) MM is good on the more timid one with little boys too, but he does have to be taller then TT and Sorin.
Turtle talk is great, along with the new Nemo ride and the revamped Mexico ride. Then there is some great stuff inside inoventions, and the light up side walk and the spraying fountain, ice cool station and Plus the WS is really cool at that age, so neat to tell them about all the different countries. Little pass ports, kid cot stations and pressed pennies make it more interesting. Really for us Epcot is a 2 day minimum place, and a don't miss.
It goes like this for us when we have kids with us.
MK
Epcot
AK
and MGM last.
Why MGM last? There isn't that much for kids, a lot of it is shows and it is very difficult to just go around and see what you want to. Much of it you have to plan it out to be at the right place at the right time.
For us MGM does have Ariel show (I have girls remember) Play house Disney ( I have a 3 year old too, but her big sister still enjoys it.) Beauty and the best show and Fantasmic. Muppet's is fun if you decide to go, but my oldest is really scared of them.
TT and RR are great, but too intense for my little ones. If they wanted intense you can do EE at AK too. MGM is really lacking for little kids and for places that you can just go to with out having to be on a schedule. Your kid might enjoy the new race show, but I would not go there just for that.
AK is a short day, it doesn't have as much. If you are going to park hop I recommend it a half day, but don't miss it. Plus it is a great place to take things at a reduced pace and walk around and really check it out.
What ever you do, don't skip Epcot. Skip MGM or AK before Epcot.
Oh and I would not bother with any EMH at any park other then MK (and maybe Epcot.) Most the of the attractions open aren't ones little kids go for.