I work for a church and we do something similar each Christmas. If you'd like, here are some tips.
1) Get your families through a reputable charity or a school family resource center, since they've usually screened them before approving them for the program. Also, cross-check your list with other agencies to make sure they're not signed up for help through other programs. We usually have two or three families that sign up for five or six different programs in town, meaning other families can't get those spots and go with nothing.
2) We put up a board featuring cards for each family, no names, but with information on each child. Sample: Boy, 9, Jeans size 5/6, T-shirt size: Medium, Shoe Size 12. Really needs a coat. Likes board games, puzzles and Power Rangers. Each person who decides to adopt a family, takes a coded card and does their shopping. The items are wrapped, marked with the family's code, and returned to the office with the card. That way the family's identity is protected.
3) We do not buy large electronic iterms or video games, bikes, ATVs, Ipods, or anything like that. We make that clear when the families sign up.