I do think this surface thing is a bit overplayed in significance. To win the French Open it is a huge advantage if you hit with heavy top spin. You can learn that on clay of every color. There are mot many of the top US players male or female who do that and would they do it more if they trained on red clay instead of green ? I don't think so. It is down to technique.
Similarly all the other surfaces have been slowed down at grand slam level which is why players like Nadal have had success in them. Yes they are still a little faster than clay and they suit flat hitters better, but it doesn't necessarily follow that kids who train on hard courts will have a big advantage over the European players if they get to play in the US Open.
I doubt whether Venus Williams or Pete Sampras ever played on grass as kids but it was their style of play not grass court experience as kids that was more important.