I am different from everyone else. If I were taking a young child to DLR for the first time, the FIRST rides would be in DCA. I would take him/her to bug's land (NOT "it's tough to be a bug!") and spent a few hours riding the Chew Chew Train, Flik's Flyers, etc. Just playing around in there, in the nice, outdoors, happy, cheerful, not scary, area.
Then once they are used to rides, I'd go to Fantasyland.
You see, the problems with Fantasyland is that so many rides are indoors. Or take you through the mouth of a scary whale. Or you just can't see all of the ride. The kids don't know what they are getting into. My son, who had been on Peter Pan many times before, had a freakout in the line for PP one morning; once you're under the roof, it gets dark in there, and he just didn't like it. He KNEW what the ride was like, but it took serious convincing to keep him in the dark line to get the payoff.
And I know there are other kids with those problems as well, I would say it's actually normal for that age range. They just don't have the experience (or they don't fully *remember* previous experiences if they've been there before) to know that it will all be worth it in the end.
And then of course if you aren't careful, you'll go on Snow White and Pinocchio, and those are (sorry, it's my opinion) just AWFUL rides. Storylines make no sense and skip HUGE sections of the story, they are dark dark dark (in story and lighting), etc etc. Get on one of those rides before the kids are ready, and you might not get them on another ride the rest of the trip.
So I say, start with bug's land!