Go with the dominant eye. But I would teach them to be ambidextrous while they are young and before habits are ingrained. It can come in handy for awkward shooting situations and especially if eye dominance changes or an injury occurs. A shotgun I would still go with the dominant eye but let them try both ways.
One thing good about shooting left handed is I am not constantly tempted to get another rifle as there is not a lot of choices out there and I pretty much have all the bases covered sometimes 3X over.
