Call me a prude, but I do not like the idea of parents buying sex toys for their children. Yes, parents should educate their kids on sexual health and wellness, but that job does not include buying things that will help them get their rocks off. This is doubly true when said child is clearly going through a tough time and thinking they are in the "wrong body". TRAs will deny it up and down, but transitioning is a very sexual thing. Sex is a big part of transitioning culture, and there is no doubt in my mind this poor 13 year old girl has some messed up views on sex already. Why mess it up further? This kid needs therapy, not sex toys.
I also do not agree that buying a sex toy is like buying condoms. Condoms prevent pregnancy and STDs. That's called being smart and proactive. You don't need condoms to have sex or partake in sexual activity, it's just a smart idea. It makes sense for a parent to buy their sexually active kid something that prevents them from becoming young grandparents.
In contrast, a vibrator's sole purpose is to provide pleasure for the person using it. Why on earth would we want to encourage parents to be apart of giving their children pleasure? If this kid is going through puberty and curious about how to safety please herself, just tell her what NOT to do (e.g don't stick food or sharp objects up your vagina) and find her an age appropiate website/book to read more about it. There's plenty of ways to get off without a sex toy, and most girls that masturbate certainly don't start out with a sex toy. And frankly, I don't know why we would want to encourage kids to start with a vibrator first anyway. What's wrong with just using your hands or pillow like a lot of girls do?