Just been checking the British Schools Cycling Association regulations. Distances as follows:
U8 - 2m, U10 - 3m, U10 - 5m, U12 - 6.5m, U14 - 12.5m, U16 - 22m. If you decide on different distances, they are a useful guide to proportionate distances for each age group.
Wouldn't recommend straight course races - even given the size of some US carparks, they'll be pretty short. If you have the manpower and resources, set out a roughly oval/rectangular course, but with one zigzagish section. This will help sort out the kids who are good bike handlers and help to balance the race between those who may have a lot of strength but less finesse.