"What I am constantly trying to get them to realize is that when I am coaching them, I am not Dad, I am their coach."
Except, you're not. You're still dad. This is why my ex-wife and I pay other people to teach our kids music lessons, even though we're both eminently qualified to do so. Likewise, the answer on this board to "should I coach my spouse/significant other?" is universally, "Don't."