It’s not that parents have more leverage, it’s that everyone knows a parent’s threats are real.
Much more fun to beat up on parents than cancer patients, isn't it?
When my kid is sick, I take a sick day. When school and daycare are both cancelled, I take a vacation day. Everybody at my work gets the same amount of both, only I'm more prone to using up my sick time and using vacation days to cover illness. Single people generally get all their vacation as vacation, and their vacations are breaks from responsibilities. My "vacations" happen on a schedule determined by school, and it's a vacation for my kid, with no real reduction in my responsibilities. "Hidden costs" indeed.
Employee B: I want to go home and relax.
As your co-worker or your boss, why do I care in the slightest whether you're going home to take care of your kid, to smoke a bowl and watch a movie, or because a new video game was released that day?
If A, B, and C are all getting their work done in the time that they are working, I think equally of the three of them, as subordinate, peer, or supervisor.
A, B, and C should all just leave at a regular, reasonable time. And sometimes each of them has reason to leave early, and doesn't need to justify it.