All true. At younger ages, DD was always an infielder because she was just better at it than most everyone else. We worked at it...a lot. At 12U, I started playing her in the OF during games, and spending a large part of our individual practice time on OF skills. Now, as a HS-aged player, she still plays IF, but because she's an effective OF, she sits so rarely that we laugh about it when that happens. The coach's kid is the stereotype that plays SS, pitches, and bats high in the order (regardless of performance). She's a bit of a trainwreck in the OF because she rarely plays there or works on those skills. The thing is that the coach really wants his kid to play in college, but her athletic ability signals "outfield" in bright neon lights. She's a good SS and often an effective pitcher, but she might not compete well at either position at the college level. However, she'd be an outstanding OF, if she worked on it.I don't think it's even a rare thing. By definition, coaches are invested. They've probably been practicing with their own kid 3x a week for years, just because they enjoy it. Even a kid who's not naturally talented will do pretty well with all the extra practice they're probably putting in. My DH only head coached for two years, and my DD would've enjoyed playing more outfield. She got to play very little, because he needed her at SS. Coach's kid playing SS, gasp! She also used to pitch! Double-gasp! But she was good, and earned it.
The same has been true for other dad coaches she's played for. The trend I've noticed, if there is one, is that some of these dads tend to be harder on their own kids. Their kids have been hard workers and good ball players.
DD played for a non-parent coach in first year 10U. He was very knowledgeable. She learned a lot from him. But he had just come through 18U and did not have a very good rapport with 9 year olds. I think for 14U and older he was probably great.
Sometimes, coaches do a disservice to their favored position players by not getting them the work they need to succeed over the long term.