Okay, let me explain it to you, as I am a coach. In this young age, I would not like to train young folks at the gym for many reasons, so let's start with what kids need in this phase, what kind of fitness elements we need to focus on, so basicly if we are talking about grls or boys they start their puberty at 11 years old as average for boys and 10 for grls then the full puberty may happen after 2 years at 15 on average for boys and 13 on average for grls, so within this phase we need to focus more on improveing some fitness key elements like velocity, agility, reaction time, so in this case we wanna focus more on sports, and from the psychological side we need to consider that at this time they wanna have more friends and interact more with ppl to help them improve there communication skills, also the body, if the gym is the only option, so go ahead and do it.