How best to talk to another child's parents about bullying?

If it is happening at school, it is best to confide in the teacher and let the teacher handle it. If it is not in the school context, then you might just consider not allowing the child to play with your child anymore. If there is some situation besides school that makes it necessary for your child to continue being around this child, then stick to the facts only. Say a lot of "I wasn't there to see it...but..." or "I don't know what my child's role is in all this, so if you find out anything more, I'd be curious to know..." etc. That will allow them to save face. If their child is a bully, they will have heard it before. If they react badly, don't worry about it - you just found out why the kid is a bully. (They may bully you if they sense you are conflict adverse, so shoulders back and stand your ground with (pretend : ) ) confidence!)