Quite some time ago, on a Qantas flight to Hong Kong, a child sat down in the seat beside me and her mother was diagonally across the aisle. I felt quite uncomfortable about this, for the precise reason mentioned earlier - I would have to accept responsibility for this child in the event of an emergency. An FA asked me if I'd move to the seat diagonally across the aisle, so that mother and child could sit together.
I was much relieved and therefore readily agreed.