Growing children will literally starve to death without protein

Everyone will die without proteins. And you can get all of the required protein from plants and plant based foods. This is not only the case for adults and children but also for pregnant people.

don't kid yourselves; if we don't eat that cow, another omnivore or a carnivore will

You know that we breed a crazy amount of animals into existence for the sole purpose of killing and consuming them, don't you? And you know that a lot of times we even throw away a lot of what's edible from the animal? No other species on earth does this.

Furthermore, even in the wild predatory carni- or omnivores usually don't kill a whole population of animals. They kill some, yes for the purpose of survival. But by far not all. And even if that happens, those predators will starve and die until more prey is available again. That's how predators and prey are balancing. Meanwhile we kill basically every animal we breed for food and we wouldn't even need them for our survival. There is no such balance. To the contrary. It is one of the major factors of environmental destruction and pollution.

while dogs, cats and ferrets are obligate carnivores and at least need meat-derived pet kibble in their diet to live

First of all that might be a reason not to get a pet. Secondly, dogs, cats and ferrets can be fed on a plant-based diet. It might not be as easy and should definetly happen with support of a veterinarian (as most people won't know for sure what they are doing), but it is proven to be possible without inflicting harm on the animals.