If it is just some random kid from the neighborhood I just say, "No thank you." I don't think that makes me uncaring jerk - just a neighbor who doesn't need a pie! If it happens to be a child I'm close to in some way (niece, children's good friend, etc.) then I'll decline the purchase but offer to make a donation to the organization (assuming I think it is a worthy cause, of course). If there are prizes involved for selling the most then I don't care at all if that child takes my donation and buys a pie (around here it seems to be pizzas and cookie dough) to help their sales.
I NEVER buy something twice for the same organization but different kids. The first one to my door is the one that gets the sale.