'Axe to grind'
So you have no desire to safeguard your girls and are happy to put them in a situation where they know there's a boy in the group (cos they will and do) and then certain subjects or issues come up and they go silent because they are uncomfortable with him being there.
There was a boy in the year above my son who was included in all the puberty talks at school. Including with Muslim girls. They were all so unhappy about it. But they didn't say anything openly. They were happy playing with him etc but there was still a line where they were unhappy.
They'd been conditioned not to say anything though and instead be silent and not voice how it made them feel.
Are these the people with an 'axe to grind'? The actual 9 and 10 year old girls themselves? Or what about the parents concerned about the fact they know their daughter is upset and being forced into a situation they are uncomfortable with and being taught not to stand up for themselves? Are they grinding axes?
God forbid there's an actual safeguarding red flag or fail... Which you are potentially on the hook for by not checking the sex of the kids - safeguarding is there to protect leaders as well as kids due to how some adults lie.
If you don't insist on checking all the girls as routine, you put yourself in a vulnerable position later down the line. You might wish to consider this. HQ can have a policy not to check but it's you who will face the issue personally if something happens at your group and there's a complaint or concern. HQ will leave you and other leaders out to dry with a don't ask don't tell type policy...