You should have your children record how far the marshmallow flew for the first 3-5 shots. Maybe get them to measure how far the catapult was pulled from its original position. This is a science experiment after all, and observations are a VERY important part.
At this stage, you may want to experiment with the materials that you used too. Try different things and see which catapult works the best!
And don't eat too many marshmallows;)