@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.
-S-