As a Mom of a 5 and 3 year old Daughters and I also Race this is what I would do. I would say yes you should race and nothing should stop you just because you are a girl! Next I would go buy tampon,pads(different Sizes) and different undies with extra padding in the croch area. Of course just her cycling shorts also. I would let her try them all out on the bike of course and let her make her own decision on what feels comfortable because we are all different and we all have different comfort levels. If it's light I think the shorts alone will be fine. The other very important factor is how long is the race/how long will she be on her bike. If it's a short time then your options are better. Give her ALL her options and she will be just fine!

Kim