Don t assume that just because a child can t sustain interest in a project, he doesn t get anything out of the experience. It s not necessary for him or her to see the project through. It s hard for them to stay focused on one task for too long.
Children have a short attention span.
Take a break. Continue the project later or another day. Don t place adult expectations on children. They ll always fail. The goal of the project should be to do something together, and to have the child feel a sense of participation.