As one who has taught and coached for forty years I give this article a hardy thumbs up.
Childhood should have generous portions of movement and play that are on the child's terms. Certainly there must be some parameters, giving a young kid carte blanche on climbing a 50-foot tree comes to mind. However, if you want to end an otherwise fun and productive session of recreation try imposing a bunch of rules on the proceedings.
One of the tests mandated by our state for physical education is the 20 meter P.A.C.E.R. test which tests cardiorespiratory endurance. Many kids dread it although they adore a game like Capture the Flag. I once asked several classes to guess which activity covered more distance and after some thought most of them realized that, for the majority, the answer was CtF, especially the "underconditioned" kids.
Keep joy in the process. Yes, it's something that adults need to take to heart. As I have mentioned a couple of years ago my father-in-law could play with our sons essentially step for step during their childhood and early adolescence until his untimely passing to bile duct cancer at age 74. He was a kid and experienced their pure joy right along with them.
Swimming and gymnastics are well-advised. While most of us take hitting the ground and getting back up for granted there will come a day for all of us that falls take on a whole new meaning.
This is a community of driven people seeking strength among other physical qualities. There is something noble about the quest for getting better. Finding a system that allows for that often calls for adherence to detail. I get that. We all have to face that in our lives, vocational pursuits, etc. Often this leads to losing sight of simplicity and the joy of recreation, of play.
Don't do it to kids and try to minimize how much we do it to ourselves. Sometimes the joy and reward is in the journey.
@Denny Phillips, in Easy Strength, the idea is put forth that children should be generalists. The "quadrants" talked about there give a good way to think about the general and the specific in physical activity.
I'll give you a story from one of my kids' childhoods that illustrates what I think is a relevant point in this discussion.
Our oldest was always very athletic, both strong and fast, and when he was young, I was an avid amateur distance runner, cyclist, and swimmer. Somewhere along the way, we heard about the Junior Olympics, found out that the first level of the competition was open to all comers, and asked our son if he wanted to go - he said yes.
To make a long story at least a little shorter, we took him to the meet, where he ran the 1500 meters (or maybe it was a mile) wearing blue jeans, a t-shirt, and some relatively inexpensive sneakers while many of the kids around him had running shorts, singlets, and running shoes - and coaches. Many of his fellow competitors had coaches.
When it was over, our son placed high enough to qualify for the next round of competition, to be held later at another location, but he was required to say, then and there at his first meet, if he was wanted to go on. He didn't really want to, and we, his parents, weren't sure quite what to do.
We were fortunate that an "elder" saw us discussing this with our son and came over to talk to us. He took us aside and told us that we shouldn't pressure our son to go on to the next round, and the reason he gave was that he said he could predict, with certainty, that none of the children who'd placed ahead of our son would still be running by the time they were in high school or college, that they'd burn out from too much training and/or too much parental pressure.
We thanked him, we didn't further encourage our son to go on to the next round, and we're glad to have received good advice at the time we did. He went on to play fall soccer and spring track in high school, and is still strong, lean, fast, and plays various sports - soccer, basketball - every week, and he lifts, too.