While her technique isn't bad (look at how she describes Dursley's day in the first chapter, for example -- when she mentions what he doesn't see), her language is at this point very naivistic, almost too simple. It does not flow naturally, and she repeats words too often. This is something that suggests a children's book to me. There are also a couple of phrases, like "he looked simply too big to be allowed" which ring like a child's language (or a grown-up talking to a child, at any rate).

But because she uses this language to point out things that normal children's books wouldn't, it appeals to us as well. The early description of Petunia, for example, is gold. As is Vernon's slow descent into madness.