This is NOT theft. It could just be a misunderstanding. You said: "However, the mother had asked to use my computer several times during their stay here, and I was not in the room to see what they were doing." So you DID in a way give them permission if you said yes, then left them to it. They may not (giving them benefit of the doubt) have realized that you had a limit or that downloading videos would use up so much bandwidth.
At any rate, this particular instance is not criminal... just rudeness or thoughtlessness. No sense in crucifying them, for goodness sake!