I agree that the latter Harry Potter books have themes which are dark but different kids are affected to more or less of a degree. My DD didn't like HP but relished Dr Who at the same age. She moved on to the Hunger Games aged 8.

You could try the Alex Rider series by Anthony Horowitz about a boy spy (junior James Bond). James Patterson wrote the Maximum Ride series about mutant children who can fly. Both are actually much easier to read from a literary perspective than HP.

Wrinkle in Time is excellent fantasy.