I did the same as Anthony, use cotton in the baby's ears with a small piece of masking tape holding it in. Worked fine. As they get older and the head expands and solidifies, you can get a small pair of shooting ear muffs or ear plugs.

Just pay attention to what is happening around you and try to keep them away from cars warming up. Also, be observant with where you watch the races from. I.E. the entrance to turn 3 is a lot quieter than the exit (where the cars exhaust is pointing at the fans).