Kids lift weight all the time, and more than most adults. Think about how often a kid climbs up a tree or jungle gym. They are lifting their entire body weight. When they piggy back one another, they carry a friends' entire body weight on their back whilst running.
If your kid shows an interest in lifting weights, let him try lifting a light weight – he probably just wants to copy your movements.
If it's too heavy, he just won't lift it anyway – there is no way a kid will keep straining, they're not motivated by ego like us oldies.
A broomstick is a great substitute for a curious kid.
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