This is not a zero sum game.

If we're looking at existing staffing levels, then sure, giving some people flexibility would come at the cost of other people's flexibility. But that premise is exactly the problem here — it's fucked up that when people who aren't parents are in need of flexibility, their employers often say that it's not possible because staffing levels are too low. But this is a problem that arises because employers are cheapskates.

They're advocating for equal treatment for equal pay. Having kids doesn't make you special.