Even animal based cat food has to be fortified, so I don't find arguments about "natural" diet convincing. I'm also not sure what nutrient cats would have to get from animals. Animal specific nutrients are things like assembled proteins, which bodies break down and reform anyway. Assembled proteins are the natural way to consume amino acids but if anything that causes problems. Foreign proteins can over trigger our immune systems causing allergic responses and auto immune disorders, in rare cases misfolded proteins can directly cause disease like in the case of prions. There's a reason hypoallergenic pet food exists and you don't see people calling others monsters for feeding that to their pets even though that seems to be the common sentiment against people who feed their pets vegan pet food.

I don't understand your point about children. If your child wants to eat just Oreos would you let them? Parents always control what their children eat, if not directly, simply through what they are able to make available.

To me it's kind of moot though because I don't think we should be breeding "pets" in the first place. But while there are still pets that need to be adopted and cared for I don't think sacrificing livestock is the way to care for those animals. We are deciding that the life of one animal is worth the life and suffering of dozens. If you are ethically against vegan pet food then you should be pro murder/euthanasia because we are making a decision to kill either way.