Life after Harry Potter

This is an old threat, but I'm resurrecting it without apology to recommend a couple that haven't been mentioned:-

1. Susan Cooper's The Dark is Rising sequence

It's quite dark in parts, but still a young adult fantasy series. It begins with Over Sea, Under Stone... Three child protagonists go on the train to Cornwall, and meet their uncle, a kind of Merlin / Dumbeldore figure.

The titular The Dark is Rising is the second book in the sequence, the scariest and best of the lot. It was made into a dreadful movie.

1. Alan Garner's The Weirdstone of Brisingamen

Quite similar to The Dark is Rising in some respects... Two child protagonists, abroad in the country, facing evil forces. Garner drew on a local legend for the plot outline, and there's a kind of dark British pagan element that is in Susan Cooper's books as well in parts.

I feel like these could be good options to follow Harry Potter in that they are as adult or perhaps more adult in tone, but still YA. They might bridge the gap to Phillip Pullman. I like His Dark Materials but for a 9 year old it is probably thematically too adult (Reading difficulty less likely to be the issue). I'd suggest Terry Prachett likewise was obviously not writing for 9 year olds, despite the covers. Ursula Le Guin's Earthsea is a great shout though.

I presume you've considered The Hobbit also.