EDIT: The original question didn't mention the age of the "young girl", so my answer assumes this could be a young child, and then later a young adult.
Assuming this young girl you speak of is a child, then there's very little that can be done by the girl in terms of martial arts during the assault. She can yell "no", kick, bite, scream, and run away. That's about it. Martial arts training generally won't do much to help her, not even Brazilian Jiujitsu. That's not just because of the size difference. It's also and primarily because of the psychology of the situation.
From what I've read on this subject, children that are raped are often the victims of relatives, authorities, and other people they know. They're usually groomed to accept these situations, so they're not going to even think about screaming. Even without grooming, they may not understand what's happening to them and won't know how to react. It may never occur to them to yell "no", scream, get away, etc.
There's an entire line of psychology devoted to understanding this subject, with many books and research papers written about it. Suffice it to say, it's complicated.
Probably the only thing you can do as a parent or teacher of a young child is to teach them where the boundaries are, what is good touching and bad touching, who can do the touching, etc. And then give them a very simple set of instructions for what to do when they suspect something bad is happening. They also need to be taught to recognize when they're being manipulated into keeping silent after the fact.
Martial arts training is relatively unimportant compared with those other things, at least for young children.
As for girls who are older and have more body weight and maturity (say, 11 years old and up), then martial arts training might help them get out of situations where a man is on top of them, pinning them down on the ground. In that scenario, Brazilian Jiujitsu is one of the best, if not the best, martial arts to know. They need to understand that their goal is to try to get out of there as quickly as possible. Chokes and other submissions should be a distant second.
Keep in mind, women's "self-defense training" doesn't generally work. This is typically taught as a single day or a few days of training, whereby women are taught all kinds of ridiculously ineffective things to do in rape situations. There are videos online debunking them. (Look up Ramsey Dewey's videos, for example.)
So this requires signing up and actually learning Brazilian Jiujitsu, and doing it for a couple years in order for your skills to become good enough to be reliable. Don't fall for self-defense courses. They're useless wastes of time, in general.
But in addition to that and perhaps more importantly, young ladies have to be taught how to recognize when boundaries have been crossed and to bring it to the attention of their parents and other authorities. Once again, most of the time it's a relative, a friend of the family, or someone else they know that attempts to rape them. And they might have groomed them ahead of time to accept it. Girls have to be taught to recognize patterns of behavior and then speak up.
That's going to prevent the vast majority of bad situations from happening in the first place. For all other situations, BJJ is probably the way to go.
Hope that helps.
