The objection is to her featuring goblins, which were commonly used in children's books in the period when she was writing (ultimately inspired by George Macdonald in the 19th century; he was rediscovered in the 1980s). Just google "goblin children's book" and see how many hits you get.

I think I know exactly which book was her main influence, as it was popular around Edinburgh a bit before she started writing the HP books - my wife got it for her kids. Nobody's ever accused that book of being antisemitic.