What is the main thing for children? To have fun! A car is always fun. Therefore, the idea of equipping a sandbox in the back of a homemade truck will surely appeal to everyone. The main thing is to make such a machine cheap and not difficult.
Put together a regular frame from planed boards, find four old tires for credibility. Silently fantasize with the cabin. The old steering wheel and parts of the front panel can always be found at a car dump. The grandfather can be adapted under a box for storage of toys. The cabin can be made more voluminous, painted, and the sandbox is ready. At least the boys will be delighted.