How do philosophers justify the ethical/moral theory that "it's wrong to use someone such as babies/kids who are mentally incapable of treating others as an end in themselves as a means to an end"?

I had a discussion with someone who told me "you need to first give reasons to why it's wrong to use someone such as babies/kids who are mentally incapable of treating others as an end in themselves as means to an end". They said;We don't have to forget something, all the moral rules have their limits, is a plant an end in itself? Is it bad to eat plants because we are using plants as means to an end which is our own survival? What if I want to survive a storm so I look for wood to make a refuge? Would I be treating the plant badly? So saying "it's wrong to use someone or something as means to an end" obviously doesn't make sense, it is not just arbitrary, you can not treat people with rules they do not follow, that's why it is not immoral to arrest a thief, and to use the thief as a means to an end, we can't treat a thief like an end in themselves when they can not function that way, knowing how to act exactly with these kinds of people is entirely a new and big problem, but I say this for a reason, nobody treats babies/kids as ends in themselves because babies/kids are mentally incapable of treating others as ends in themselves, babies/kids are not free men and can not be treated as ends in themselves because babies/kids do not respect freedom, we know that. A thief like I said before goes to prison because they were free, but violated freedom, a kid is a responsibility of their parents, they won't go to prison because they are free men in process, their parents got to answer for them, so that idea of "don't treat babies/kids as means to an end" wouldn't apply and wouldn't make sense.Does anyone have any arguments against that? via /r/askphilosophy https://ift.tt/3n5JK0l