There may be a point at which humans are probably overburdening the planet through numbers alone (I don't think the growth is purely exponential; I wonder if they could reach this point in practice without toppling the food web, destroying the physical environment, or running into other population-limiting factors like plagues). Still, humans contribute so disproportionately amongst themselves to environmental destruction that overpopulation feels like a bit of an oversimplification. The fact that a lot of Malthus-style overpopulation rhetoric has been pushed by many of the worst environmental criminals as a means of social control and racial eugenics (and deflection from that disproportionate culpability) also makes me suspicious of advancing it as an argument.
I am not actually antinatalist, but I believe all humans should have the right to leave this world, unhindered. We don't, so that affects my feelings in practice. I care about children a lot and know from personal experience and study that they are more defenseless than any other human beings, in part due to being undermined, disbelieved, and pushed around by people who are totally indifferent to the power they wield over their development. So, I sympathise very greatly with antinatalists, and think that wanting humans to do more to protect children, especially at this point, is probably naïve denialism on my part.
I have mixed feelings on the topic as a whole. I think it is possible to lead a comparatively happy existence with most diseases and disability, with that not being the major cause of suffering. Actually, I have a disease most people on here would say is pretty unacceptable (although my parents did not know about it until I was an adult). Of course, it's nowhere near as life-limiting as the more severe forms of osteogenesis imperfecta... It's pretty difficult to have mixed feelings about this case.
Nonetheless, I think the parents' motives here are likely terrible, and I doubt they are willing to have a serious conversation about it with their daughter, who cannot advocate for herself at her age but might quite prefer not to have suffered so much. The alternative is now that Zoe is alive, she may not prefer to die because she is one of those people who ends up on the whole thinking life is intrinsically better, but she will suffer and suffer anyway. That is also cruel. So, I also cried for Zoe. Breaking bones just through developing. I wanted to work in a field very related to this, and it's things like this that partly caused me to lose faith in what I was doing. I don't want to be an enabler.
(I'll note that, due to the nature of my mutation, I would be transmitting a far more severe form of my disease if I reproduced; there are a million reasons for me not to reproduce, such as said environmental destruction, but even if those didn't exist, I would consider that a dealbreaker. Because I can tolerate my disease and most people with it are happy does not make it responsible or necessary in my opinion to have a child with this disease.)