If you are hosting it, maybe you could try taking the initiative? When it is coming to an end and people are getting ready to start going, get up and say " OK everyone, lets all help tidy the toys away." You can direct this at the children, but loud enough and clear enough that the adults pick up on it too. If they still don't help, just say quietly to them "Oh, do you mind picking up x, y, z before you go?" Most adults wouldn't have any problem with that.