[Hangs head in shame.] My childrens' first WDW attraction was... the Swiss Family Robinson Treehouse! We started touring in Adventureland, and not only was it the first attraction you come to, but I wanted to slowly build the excitement level for the kids. I was afraid if we began with something like POTC, they'd balk at less exciting attractions, so we did them in more of a least-exciting-to-most-exciting order in each land.
That being said, if you want the first attraction to be classic and memorable, I think that Dumbo, POTC (if the skeleton pirates won't be too scary for them), Peter Pan, or It's a Small World would be great "first attractions" for a young child.