It's completely academic where the mother/guardian of the child was sitting at the time he was misbehaving. The salient point is that she was not engaged with what he was doing or how his behaviour was impacting on others. Had she been watching him, she'd have had the opportunity to intervene before someone else had to. And the "circumstances" of the mother and child are not the responsibility of the OP or anyone else for that matter; they are entirely the responsibility of the mother/guardian. OP only had responsibility of the safety and wellbeing for the GC that she was looking after that day, and if that involved checking another child's bad behaviour; so be it.
To think that London, or anywhere else for that matter, does not belong to any one demographic