Home Depot has project kits and I think lowes does too. For a five year old I wouldn't let them independently. Sometimes you can get in builder classes and build the project right there and sometimes you can bring the kit home. I think my sons did Home Depot wooden nascar cars at a community event. Oh and you can order little pine wood derby car kits by searching cub scouts. That does need supervision because you cut and sand down a block of wood into a car shape. If you have a scout shop around you could go and buy one there.
