This sounds like a great approach.
Hopefully the parents won't be instilling the kind of messaging that we can see on this thread at home e.g. that anyone who won't actively use they/them pronouns is evil and abusive.
As long as that's not happening, the child won't be primed to look out for this and will be completely oblivious to the position you have taken.
The only exception I can think of to this is that perhaps you not "correcting" children who say she/her might be noticeable. It may be worth having an idea about how you would approach this, should it arise. On this point, you could speak to the head and ask for direction in line with KCSIE. Here's what the guidance says:
"279. The school or college should also make sure that children and their
families are aware that while the school or college will appropriately sanction any cases of bullying or harassment, and take a strong stand against bullying, the school or college must also be conscious of the rights of pupils and staff in relation to their religion or belief. However, schools and colleges supporting social
transition might consider discussing options with pupils and staff such as using names instead of pronouns"
Given the children in the class are 9, it's unrealistic and unfair to expect the children to stop using 3rd person pronouns naturally and instead use a name. Maybe the head would be happy with something like:
"Not everyone will use they/them pronouns. That doesn't mean they are trying to upset you."
Just as importantly, the head needs to be ready for some adult conversations on this. Especially if the parents of the child come barrelling in with any of the ridiculous hyperbole on this thread.
If the parents (or any staff members) do this, the head needs to confidently explain to them that the school doesn't teach the children about non-binary identities because it follows the statutory RSE guidance when explaining facts about sex and biology to children. From the guidance (bolding is mine):
70. In teaching this, schools should be mindful that beyond the facts and the law about biological sex and gender reassignment there is significant debate, and they should be careful not to endorse any particular view or teach it as fact. For example, they should not teach as fact that all people have a gender identity...
Lastly, the Forstater judgement has already been mentioned above. Personally, I think the most important part of this judgement is not the protected right to believe that there are two sexes but the protected right to not believe that everyone has a gender identity (screenshot below). This means that nobody needs an alternative belief system to justify why they don't use they/them pronouns actively. You're protected in law by virtue of your lack of belief in gender identity and the school can't force you to pretend you have one no matter how much the parents might stamp their feet and call you evil for not being a believer.