Stitches Great Mistake is by far the worst decision WDW has made in changing a ride (in my opinion), but then again, we loved ExtraTerrorestrial.
It is much like the Alien Encounter previous ride in the aspect of the same set up where you are in a circle in the dark around something in the center. Remember, this is Stitch when he was still 626 and busts out. At one point Stitch belches a horrid chili dog in your face, whispers in your ear and steps on your shoulders.
If your kids are afraid of the dark, then I wouldn't recommend it. Personally, I have a problem recommending this ride to anyone, regardless of fears of the dark. But again, MANY people love the ride. It is much like AE but toned down to have stitch instead of an Alien with vile smells and less excitement.
Em