Look at the other juniors at the dojo. At this age being among her peers is important. My daughter was 12 when we started and was one of the younger students but since we practiced together she felt secure enough. The choice of martial art depends a lot on a child's personality too. She found Karate boring, too many repetitive drills. Judo, which was my first martial art, was a bit rough and tumble for her taste. Aikido was what she enjoyed and the defenses against being grabbed, tackled, etc. should stand her in good stead.