This is a question I have written about before, but a good one. I think you cater your list to the kid. Our first year we played a single deck, but we were fortunate to be in a meta where that deck was competitive all year. It was a simpler (less rock-paper-scissors) meta, admittedly. (2014, we just played Plasma all year) When I reflect back on the Virgen/Yveltal/Plasma meta that dominated the format through 2014, it felt like a pretty boring meta for deck building, but a pretty good meta for skillful play because no matter which of those 3 decks you played, you felt like you had a decent chance to win against any of the other decks. We never felt like we had "made the wrong deck choice" (although we had zero expectations) and we played more or less the exact same deck all the way to the World Championship.
Today, we are definitely in a standard meta that actually, to me, feels pretty skill-based and I think you could play MGard, Yveltal, Fighting, Greninja, or Volcanion and with some luck (of which it feels like we have none) see great success.
I have written before about how we nerf deck lists for my youngest. I think you have to look at your kid and make the decision. My oldest is more comfortable draw/passing and starting a bad hand. My youngest is simply not as good at resource management. He will play every Shaymin and Trainer's Mail he gets his hands on. And if there is one thing he hates, it is starting a turn where he has played himself into a corner so he is forced to essentially draw/pass. So we have found that cutting a tech or two to improve the consistency of a deck is good for him. He feels good about losing if he feels like "he lost", if you take my meaning. He wants to feel like the other kid beat him, not his deck. He would be miserable playing Greninja because if he just got donked a few times the non-interactivity of the game play would just put him on tilt.
I have said that a lot over the years, but it is getting better. He complained vociferously about drawing dead 2x with MGardevoir during London and I sat him down and said: Yo, this deck has a million draw cards. It does not draw dead unless you make it draw dead. And then he went off and I never heard that complaint again.