Many people think "gendering" a child might be bad.
That means: Teaching the boy role to a boy might be bad. Teaching the girl role to a girl might be bad.
No one considers that teaching the child role to a child might be bad.
I think the child role is more impactful (for good or bad) than a gender role. There's a much larger difference between young children (e.g. a 5 year old) and adults than between men and women. I'd estimate that under 20% of the adult/child difference is due to the child's learned social role, but the social role is still a big factor.
The child social role has a lasting impact after childhood. People don't just forget being children. And people transition in stages where they e.g. put effort into differentiating themselves from a child. When reacting to a former role is a significant part of one's life, then that role is still impactful.
Adults put substantial ongoing effort into avoiding being childlike. It prevents them from doing much learning. They don't want to be beginners or learners because that's for children. It also prevents adults from having some types of fun. Curiosity is another childish trait which adults suppress a lot of.
Most people are mostly respectful of both genders. They don't have a significant grudge against either gender. By contrast, people are frequently hateful of the child role. They dislike and mistreat children routinely. Teaching your offspring a social role that you don't respect, then disrespecting them for years, is a nasty system. People think children are stupid and use the fact that they do child social role behaviors – while adults don't – as proof.
> They dislike and mistreat children routinely.
What do you mean by this?
By "mistreat" I guess you mean coerce children to do what the adult wants them to do. Is that correct?
By "dislike" I don't understand what you mean. Do you mean something like what you write at the end: "People think children are stupid ... " and dislike them for that reason?
People think children are annoying. They aren't friends with children. Children don't like and do the same stuff as them.