It's also emotional abuse as defined in KCSIE. It's imposing developmentally inappropriate expectations on children and imposing a belief system on them, not allowing them to express themselves naturally. All of these things are very specifically identified as forms of emotional abuse (if persistent which imposition of wrong sex pronouns and expectations that children pretend they can't see sex is)..
Adults who are full on into genderwoo can't stop themselves using sex-based pronouns as has been shown in many open justice court cases where the TRAs desperate to harm women who spoke up about reality couldn't themselves do it.
If they can't do it when they're trying their best in court, expecting a primary age child to is not just unreasonable it's abusive.
Also, sex is important for safeguarding, training children out of the instinctive ability to accurately sex people is not kind. Sex matters for safeguarding.
In addition, if you teach children that their perceptions don't matter, and the feelings of other people must always come above their feelings then they will not be able to disclose abuse.
It's all so unhealthily one way - unhealthy for ALL children involved. As PP has noted, respect needs to be a two way thing and friendship relies on MUTUAL respect. That means accepting, if you're trans, that other people see sex and respecting their right to express themselves naturally just as they respect your right to change your name, dress as you wish and consider yourself the opposite sex. Expecting that the other children spend all their time pretending around one child is abusive.
It's fine to ask children not to constantly state that a child is fat and be sensitive to that child's feelings, however it's also not ok for adults to expect them to pretend the child is thin when they're not. It's fine to ask children not to tease or bully a child if they're not very good at maths, however it's also not ok to expect all the other children to pretend they come top in every maths test and are a child genius.
All the people saying this poor child should subvert their observations for the sake of an ideology should read this: Four year old boy joins Church of England primary school as a girl
It's not kind to the trans child - as who will want to be friends with someone where you can't speak the truth about something as fundamental as sex, where the accommodation is always and only one way? That's not friendship, that's an abusive dynamic being enabled by adults.
You cannot force children to be friends with other children and they won't want to participate in 'friendship' which is so woefully unbalanced because the other children matter too, their observations about the world matter too, and children are - in my experience - finely tuned to injustice, which this is.