I am an adult who learned, late in life, how to be able to get the courage up to go on some rides (thanks to my DS, who insisted that WDW wasn't as much fun if mom was sitting out most of the rides).
My observations:
People who've always been comfortable riding coasters/thrill rides forget (or perhaps never knew?) how hard it is for people who are fearful or highly motion sensitive. I say this because IMHO, BTMRR and EE cannot remotely be compared.
BTMRR is a great getting-to-know-you kind of coaster. After I finally got the nerve to ride Splash, and after trying Goofy's Barnstormer a couple of times and determining that it really wouldn't be the death of me afterall, I got myself up to BTMRR.
Each subsequent time on a given ride is easier than the last, especially as I say if you don't take to thrill rides naturally. I now ride Splash, BTMRR, TT, and love them (NEVER thought I'd be able to say that).
Still -- EE is a different beast. I did try it once, and it is possible that I may try it again, but there's no comparison between it and BTMRR. The "fast" is MUCH faster, the banked curves are much more intense, and of course the whole dropping backwards thing.
My advice, if it's possible, would be to allow a child to try other, milder rides (Splash, BTMRR, TT) and if they love 'em then by all means try EE. The reason I wouldn't advocate having a youngster try EE as his first coaster would be that it could be VERY hard to get him on other rides afterwards if he doesn't like it. (Someday, maybe I'll tell you the story of talking DS, when he was 5, onto Snow White and then never getting him on another dark ride that trip

)