Doesn't matter if the kid wanted to do it or not, if he did there are progressions that should be followed before loads are added. As with anyone, joint integrity and strength should be first, can he do push ups, pull ups, etc. My son wanted to be like "daddy" when he was 5 so I started him out with movements like pushups, he didn't progress to loads until he was 9 and didn't start squatting until he was 12-13. He starts freshman football this year and while isn't biggest, he is amoung the strongest and fastest. Progression with kids is a MUST. You also have to understand the mentality of kids, at 4 years old (kid in the video) its not a matter of "wanting to do it", at that age they want to mimmick or, like my son "do what daddy does". Almost everyone wants to be pround of their kid but you can't push a kid. I guaranty this kids going to have some physical problems when he gets older. Joint problems, muscle pulls, etc. Notice I don't say tears, that would be rare for someone that young. There's also the issue of growth spurts, will he develop osgood slatter that effects muscle and joints during adolecences? Not always but it does happen. Only a few things to consider when training a child/kid.
