Different sports have different standards. That said, I think we Americans think that kids shouldn't be lifting heavy weights. If you were like some guys I know who grew up on a farm, wrestled, did judo or girls who did gymnastics, then you spent lots of time hefting weight around. Miguel Torres, widely regarded as the best 135lb MMA fighter in the world said that he "...can remember my soccer coach when I was 7. He made each player pick a partner and then run a mile with him on your back. We had to run through hills, dirt and sand. I was 7 years old, and he was training us like we were in the military. I looked at it like it was a challenge. He'd have us do 1,000 crunches. I'd get to 500 or 600 and have to stop. Being small, I didn't know I couldn't do it."
Kids can do things that adults think they can't. Get a 10 year old farm kid and they are used to long, hard work moving around heavy objects. Wrestlers and judo players start at young ages (as young as 3 or 4) picking up and tossing around human beings. Gymnasts do things with their bodies at young ages, often starting before 5 that are challenging as an eternal destination with 7 levels. Ever seen a ballet dancer? Little girls get into position balancing on their toes that put thousands of lbs of pressure on their joints.
Then we have what foreign kids do. At 8 or 9 after several years of day in day out structured training, Thai kids are fighting professionally in full contact Muay Thai. They are unlikely to retire until their in their late teens or early twenties. Then, for money, many transition into pro boxing.
Conditioning matters. People realize this and for kids it is starting to become more prevalent at younger ages. The things you would have them do with the TRX are likely safer and easier for them to scale and adapt to than what others would have them do. Just ask yourself how old you were when your PE teacher or coach first had you do situps, pushups, pullups, climb a rope or those animal crawl and walk exercises. If you're like me, you were in the First Grade. Now think about how old these kids are and how long they have been in a structured wim program.
I only plot world domination because the cat in my lap tells me to.