That's a good point. The first books are children books, but the series gradually becomes YA. I'd say that by book 5 it's YA already. The end of book 4 could also be upsetting for little children.

That discrepancy in tone causes some problems, like for example Hagrid giving Dudley a pig tail. That worked great with the dark but whimsical style of book 1, a style which reminds me of Roald Dahl's children stories. However, when you look back at that action from a grittier, more realistic point of view like the one the latter books have, it seems abusive of Hagrid to do that.