Without knowing the kids, or their personalities.. it makes it tough - congrats on doing this.
When I've done this in the past - me being kid-agnostic - i tend to ask kids around the same age group in a store I'm in what they might like - helps to narrow it down a little. (make sure the mom who's with the kid knows why you are askig)
th 15 year old girl, would probably want clothes -maybe a gift certificate to a reasonably priced clothing store... (where she coould get something for 40 dollars)
the 13 year old boy - sporting equipment or some sort of handheld video game?
The 11 year old girl - books from the young adult series of books at barnes andnoble?
the 9 year old girl - a doll of some sort
Check out
http://www.gifts.com/ - they might have some ideas to steer you in the right direction