As you say, it is just not practical to be hyper attentive with kids 24/7. You are a human being and life demands you to provide for kids as well which may require you to take your eyes off the children and on to other work (such as burying your head in your tablet). I can understand that the stranger was concerned and it could be triggered by his own life experience (may be he himself or someone he knew had lost their child in similar circumstance). I don't think you would forgive yourself if it did happen as we naturally blame ourselves for most things when it comes to kids but that fear should not cripple us from living our normal lives!