FANTASYLAND in the magic kingdom/
its a world of princesses, and non-scary rides galore, they'll have a blast there.
also make sure to stop in cinderellas royal table for a great breakfast with all the favorite princesses. (and prince charming, too!)
like you said, the "mountains" out. (space, splash, and big thunder.)
but Goofys barnstormer is a little-kid-friendly coaster in toontown in the magic kingdom, that is the perfect first coaster.
also try the strategy of "working them up" to a big coaster or rideuntil you find their limit.
example:
first try dumbo, if that works move on peter pan. if that works move onto something else, and just see what they can handle.
There is a TON of things kids that age can enjoy, and remeber that the "rides" arent really what disneys about! theres the parade, the joy of just sitting and eating ice cream before lunch, swimming at the pool, just the little experiences count.