Grooming, the way I understand it as something negative, is if someone treats a child (in this context, at least) in a certain way with the goal of having that child be sexual, in a way that would not be natural to the child. With 'natural' being a vague term, I'll add, in a way that is different from how the child would normally act and which doesn't spring from what the child wants.

While I think the word has its place, I dislike it when it is used inaccurately or imprecisely. Being nice to someone with whom you would like to be naked together, is not grooming if the niceness is sincere.
I find it difficult to judge your example of someone who "...used sex toys to groom the children", when taken out of context. If the adult used sex toys to introduce ideas of sex to children who had no interest in it beforehand or during, then that could be grooming.