Bullying By Robin

I think in cases like this there is no substitution for first hand knowledge. Put yourself in a position to observe first hand how the children interact. Act based on what you see.

Bullies are apt to make up any story that will get them out of trouble. (All children are, really...)

As for asking the other kids about who is doing the bullying, that's great to get an idea about who is doing it, but before you go to the kids who are accused, you need to have a corroborated complaint of an incident or first hand knowledge. Anything less, and if the parent gets involved, be prepared for a round of "Well, *my* Timmy could never do that and you have no proof!"