Years ago when our kids were small, our neighbors had two sons about 8 and 9 or so and two younger children, under 3 years.  The parents would often be gone for a full day at a time, leaving those older boys to watch the younger ones, saying that "the Lord would protect them from harm".   It scared me to death to think of all the things that could have happened in that situation.  But maybe my faith wasn't strong enough, because there was never a major incident.   Anyway, I agree, I wouldn't be comfortable leaving a young person under about 14 or 15 alone looking after younger ones.