Is judo harmful for girls

No it's not. It's actually very good excercise, having components of isometric, aerobic and mechanical excercise in it. It's also a means of improving restraint, coordination, balance and calm. There are also meditative aspects of it. Most of the martial arts try and find a way to equalize body size and strength distribution, so that girls can perform as effectively as boys, perhaps using somewhat different techniques. There is very little danger of harm as well.