I suppose it's not different to calling a child or children, "Child", or "Children", out loud, which sounds old fashioned, stiff and formal by today's standards but it wasn't uncommon in earlier (?Dickens') times. If accompanied by a kind & cheerful manner, might be acceptable.
I'm all in favour of doing away with gender stereotypes but from what I've seen of pre-schools (which in my day were Kindergarten), they are run that way anyway, children are children and they play with what and whom they choose. If parents want their kids to grow up to be happy and fulfilled, what's the problem? Only a problem if people start denying there are girls and boys which is ridiculous, there are and always will be and many of us like being boys or girls.