I mean, she was a literal child. Or a teenager, which can be called a child with attitude. Her being childish and her help causing more harm than help is pretty common for teenagers finding a cause for the first time and trying to tackle it on their own.
Anyways the books in general fall apart as they get into the last novels. As children's novels Harry Potter works fine. But as you try and take it more seriously, it starts to fall apart hard.