The weird thing about this debate is you have people on both sides who seem incapable of looking at it in any other way than that theirs is right, morally. Either meat is definitively murder and eating it is very, very wrong, or vegetarian protein is Not The Same and depriving a child of animal based food is very, very wrong.
Why not just accept that some people regard us as apex predators within the food chain, so are happy to eat meat as part of that cycle of life on the planet as long as there are good welfare standards and no suffering occurs; others think that as animals are sentient, we as thinking, reasoning creatures have a responsibility to avoid causing unnecessary death, even if the animals themselves are exempt from that requirement; and others just really hate the concept of dead flesh being part of their diet? And that all those positions are perfectly reasonable, their kids are no more likely to eat junk than any other kids, and the parent in question is probably doing what we all do - muddling through in an effort to do their best?
Someone who never feeds their child anything but trash (and I don't mean the occasional treat junk festival, I mean as a day-to-day diet) is being irresponsible. I'm also not too keen on people who don't give a monkeys about factory farming, because it's an unhealthy, environmentally damaging and cruel way to source food. (And I've never noticed the people who don't care about that being massively politically engaged, either, so the "people before animals!" cry doesn't make sense, from where I'm standing.) But the presence or otherwise of meat in that diet isn't the deciding factor.
Finally, if cats figure out opposable thumbs, lions will rule the planet. They'll probably make sure we eat an organic and balanced diet, but I fear humane abbatoirs aren't likely. They'll want to play with us first.