Coach's kids won't get any preference in athleticism, but they will likely get a better grade for their knowledge of the game. Dad has probably been watching game film in the living room since he was born, and the kid was probably getting familiar with blocking schemes around 5 years old. You can usually rely on coach's kids to not get 15-yard personal foul penalties, miss a blocking assignment, target a player, or have bad technique. They might get overpowered, but they will be in the right spot.
Same thing as the band/choir/orchestra director's kid that's been playing their instrument 8 years longer than everyone else started, they're going to be better at it and probably rightly deserve more attention.
