Our two are 4 and 6 and love flying J (and sometimes even F). But they also do their fair share of Y, so they know its a treat. As long as they have something to watch and something to eat they are happy, and they are also well behaved.
Commented in the chemist that I needed the receipt to claim some rather pricy medication. Got home and found the receipt they gave me was the eftpos one, and not the proper tax invoice. Why aren't they just handing it over by default?!