Uncharitable kids

My boys are very sweet and thoughtful in general, but when I suggested that they take a little money out of their allowance stash to donate to a fundraiser at school, neither wanted to. I think a large part of this is they want their finances to be even (yes, we have a competition problem), so if one donated something he would have less. When I pointed out that they could both donate the same amount, they began talking about the coveted toys they are allegedly saving up to buy. Finally, they begrudgingly said they might donate a dime or quarter. I was very disappointed in their reaction, as we have tried to raise them to think of others. My husband and I have talked to them about how lucky they are to have the things they do, from basics like house and food to perks like toys and vacations that many children in the world don't have. They help us pick out gifts for Toys for Tots each Christmas and we have given food and winter coats to charities. I know young kids are naturally self-centered, but what else can we do to instill a spirit of giving? I am aware that some parents lay down ground rules requiring kids to give a set portion of their allowance to charity, but I'm not sure I want my boys to give because they HAVE to... I'd like them to give because they WANT to. Anyone have ideas?