If the cars are going to be withing 200 or so yards then a handheld FRS should work fine. I have used them several times on road trips. If you need more range then GMRS would probably be your best answer. The license you get from the FCC extends to the whole family, not just you.

Ham will give you the best performance but everyone will need to get a license and know how to work the radios properly.

Also, set up a rallying point each day, someplace close to where you will be.