Giving is a serious act of worship, but there is no reason why it can't also be fun. Why not have the children designate a goal for a particular project. Make up some kind of goal meter. It can be as simple as the conventional thermometer or a picture that is colored in, bit by bit as the offering nears it's goal. The children will be overjoyed when they reach their goal. If you set a time limit for reaching the goal, you can increase the goal if they reach it early.