I think it depends on the child and adult supervision. My daughter has been around Yorkies since she was 3 and we've never had any issues whatsoever. She's also been around small dogs since she was born and how to properly handle then and act around them has been drilled into her head, so it's like second nature to her. Likewise, I've seen some much older kids that I think could probably endanger a moose if they had one as a pet.
Realistically, many breeders simply will not place a Yorkie in a home where there are young children. Their dogs, their rules, I always say. Can't blame them for trying to do what they feel is in the best interest of their dogs.