gift card to toys r us.
I wouldn't get her a gift card simply because little kids love to open presents. A birthday card with a gift card inside isn't nearly as fun. Get her something like a barbie, or my little pony with some little accessories. Give the parent a gift receipt so that she can exchange it if need be.
She's old enough...
Go to Amazon and search toys by age group and best selling for ideas. Also talk to her parents and see what she likes but does not have.
A baby doll.
I've never seen a little girl not love dolls. Buy her a doll and give her some cash/card to buy the doll clothes.