Although many books and expert advice is available for selecting opening for kids but here I would like give a bit different angle for coaches.
Beginners should play openings in which the center is equally shared. They should not choose those which give up the center temporarily and try to create pressure on opponent center with pieces. As kids are not so strong in tactics they often failed to handle the positions (general observation).
Opening repertories must include openings which produces different sets of pawn structures though we are not going to teach them pawn structure’s pros and cons in the beginning but would like them to be familiar with them so they can understand it better on right time. The coach’s task will become very easy while teaching as they may have already faced some painfull losses. For example the Four Knights Scotch Game, where White is targeting black’s hanging pawns while on the other side it is an asset for Black if maintained. Playing different pawn structures will also helpful in developing strategic skills.
Opening should be with reasonable amount of tactics, and I even prefer opening repertoires with at least one gambit because the aim of the opening repertories is not to give them just sets of moves but aim is to improve basic skills. I believe that kids can learn faster with playing real games rather than solving puzzles. For example to improve tactics, it is better to play/study tactical games more rather than solving puzzles. Moreover they like play chess more rather than learn so the coach should focus on how they can teach them through real games instead of theoretical boring sessions.
Openings must contain principles like king safety, rapid development etc. We cannot guide kids to play flank openings.
The whole discussion is focused on developing different sets of skills in child, not on giving them few moves to play and memorize them.