During a growth spurt, or "peak height velocity" (which occurs around age 12 in females and 14 in males), kids experience temporary bone-weakening, muscle imbalances and tendon tightening, and are at increased risk for overuse injuries. This is why it's crucial for children to train under the instruction of someone qualified to assess their biological age, as opposed to age in years and months.
Now, for my two cents: I say let them run around the yard some more. They have the rest of their lives to get buff.
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