Our new giving project is relief aid for the persectued Christians in the Congo (via Samaritian's Purse). This one really drew Joel in as he loves aviation and missionary pilots help deliver the food, blankets and tents. I think it is especially great to try to find something that your kids can relate to or visualize in some way.
It was great to put our loose change in the little cardboard bank that Brenton made me for a Christmas present. We label it with our lable maker, so it's interchangeable. The kids put some of their allowance tithe money (better known around here as "Jesus Money") or even some of their allowance spending money towards the project. We don't collect very much, but the kids get excited about how we can be even a small part of God's work by giving.