I have had a talk with my boys about this kind of stuff. I told them that some people are mean and might hurt them so never to go off with people you don't know. Even if they have a dog or offer a lolly or something yell at them that they don't want it and run off. Every now and then I reinforce it so they don't forget.

Is this Lydia a friend from school that was on the bus? That is a bit trickier if that was the case but the bus driver shouldn't let your child off without permission from you, either verbal or with a note!