Earlier quoted context omitted.
The problem is, people without children are told (or required) to pick up the workload left behind when the coworkers having children taking (multiple) weeks off. For once or twice that might be okay, but if became a chronic problem, no wonder they are not happy.
Yeah that's the "they have x, why don't i?" attitude that causes a lot of problems. It should be an "everyone should have x!" attitude instead. At the end of the day, everyone's human, so there should be a little bit of slack in the staffing - long term - so that when someone's unavailable, nobody is drowning.
I once as a very junior programmer asked a lead why we kept planning 75% of our capacity every month when we were hitting - and exceeding - our deliverables like clockwork.
Then one month we had to do a death march to build features that had some $$$ tied to them. Did I ever learn why a little slack was critical.