This thread should help you.
http://www.disboards.com/showthread.php?t=1671441
There may not be a lot for children of those ages at Epcot, even without autism. A lot of the attractions are long and may not be interesting for either child.
What kind of stimulation your child with autism likes or avoids will also have a bearing on whether he would like it.
3 attractions have height requirements, which the 4 yr old might make, but the 2 yr old would not/would probably not:
Mission Space 44 inch. This is a quite turbulent ride, whether you do the green (more tame side) or the orange (more wild side). It simulates a rocket trip to Mars and includes loud noises, some lights flashing, alarm noises, as well as a lot of motion.
Test Track 40 inch. This is not that rough, but includes tests of driving over rough block roads, swerving, sudden braking and then it looks like you will crash into a wall (just after avoiding a collision with a truck). That is followed by a high speed trip around an outdoor track.
Soarin 40 inch. This is a simulated ride on a hang glider, so you are high up in the air, with legs dangling. The last scene includes fireworks.
Spaceship Earth is a fairly dark ride. It doesnt have any loud noises or fast motions, but is quite long - without looking it up, my guess is about 20 minutes. (once you are on, you cant leave until it is over)
Ellens Energy Adventure includes some dark area, some loud noises and dinosaurs in a natural looking environment. The attraction is very long - the whole thing is about 45 minutes with no way to leave until the ride part is over.
The Seas with Nemo and Friends - depending on how your kids like fish, this may be a good spot to spend a lot of time and may be the only place you visit. There are 3 parts to it:
Finding Nemo ride - this is a slow ride through a dim environment with appearances by Nemo and his friends. There is one point where you will see sharks, but other than that, it is very tame and a nice ride.
Turtle Talk with Crush - many kids with autism have liked this. Crush is the Sea Turtle in the Nemo movie. He appears on a huge screen themed as a window into the sea. Kids can sit on the carpet in front of the theater and Crush will call on kids who raise their hands if they want to ask him a question. It is cute and sweet and even the little kids seem to enjoy it.
Aquarium - you can walk around and see fish swimming in a large aquarium viewing area. There are also other things to see, like a Manatee viewing tank and small tanks where you can see smaller sea animals like seahorses and sea urchins.
The Land includes Soarin as well as 2 other attractions:
Living with the Land - this is a slow boat ride where they talk about farming and how food is raised. Part is a dark ride thru some environments like the rain forest (be aware of simulated rain, some giant bugs and other crawly creatures) and a plains (with lightning and thunder). The last part of the attractions is a trip thru greenhouses (it will be very bright in there), which includes a ride by some fish tanks and some alligators (or maybe their are crocodiles). Just guessing that it is about 15 minute ride.
The Circle of Life - this is a movie featuring a few scenes with Lion King characters. It may be too long and talky for your kids. Not scary, but does include one load roar by Simba that can startle people.
Journey Into Imagination with Figment - this is a slow moving ride about imagination. It includes some dark points, a skunk smell released, a horn from a train and then train sound passing you in the dark and a loud noise and a puff of air just before the finale.
The current 3D movie is Captain EO, which is loud, pretty dark and has some scary looking aliens. you can get splashed at one point when an alien sneezes on you. The floor kind of vibrates with the music.
FUTURE WORLD also has 2 interactive areas called Innoventions East and West. These are quite noisy and probably have little for children the ages of yours.
World Showcase includes a few attractions, but is mostly walking around and looking at the different countries. Attractions include:
Gran Fiesta Tour Starring the 3 Caballeros in Mexico - this is a slow boat ride thru mostly dim scenes . Nothing scary except be aware that the last scene includes simulated fireworks.
Maelstrom in Norway - this is a mostly slow boat ride in a dim environment. It includes some trolls who tell you to go away and then send you backwards over the falls. You may get splashed as you get to the bottom of the falls.
Reflections of China in China is a movie that takes place on screens all around you. There are no loud noises, although the volume in general is high. you can easily leave if you need to during the movie. This is a standing movie, but strollers are allowed.
The American Adventure in America includes movies and audioanimatronic (human looking robot) scenes. There are 2 points with simulated fireworks - one toward the middle and one at the end). This is a long show - almost 30 minutes, but you can exit before it is over.
Impressions de France in France is a movie that is about 12 minutes long. You can leave early if you want to. There are no special things I can think of that you need to be aware of.
O Canada in Canada has a similar set up to China and you can leave early if you want.
Epcot has a station called KIDCOT Stations in each World Showcase country where kids can do an interactive project with the CM at the stop. The projects involve coloring.