When I said nothing that happens before puberty matters I meant in terms of knowing how good or bad a kid is at sports. I have spent a ton of time around kids' sports and saw plenty of kids who, like the OP's kids, were very impressive at those ages. By the time they were in high school they were nothing special. Many of them were still playing sports. Many had moved on.Some were very decent high school athletes. A tiny minority went on to compete in college. The only one I know who got serious college money for running was not really interested in running when I first met him. He was in an after school basketball program I was running at the grade school my kids went to and this kid's interest was basketball. Later he found that he could run really fast.
Psychologically, for lack of a better word, I agree with you. A lot can happen before puberty that has a ton of influence on a kid's later sporting life but frequently not in a way that furthers it. The OP may not recognize this but he's looking to monetize his kids' athletic abilities at an age when he doesn't even know if they have them or are just ahead of the growth curve. No one should be thinking about getting scholarships for 6 and 8 year old kids and if they do they should suppress the thought rather than articulate it. Then he talks about grinding through the process and hard work. Really? We're talking kids' sports here, not peasants on a commune needing to meet their grain harvest quotas. There can be a time to work hard on your sport but it's not until you're well past your age registering in one digit.