Dad, the kids and I went to the Hawk this year and have been to the fall vintage races at Road America in the past. My daughter has been going since she was around 5. There is no pressure for either kid, I just ask if they want to go and they always do. They love looking at the cars in the pits.
The guys in the pits are very accommodating.
Even when she refuses to sit in a car because it is a Ferrari (the arch-nemesis of the Cobra):
This was the boy's first time near a real race car.