Hello and welcome to the forum!
I'm by no means an expert on the Church's youth policies, but here are my thoughts:
1) It's my understanding that [generally] any child who WILL be turning 12 during the current year (e.g. 2024) is automatically advanced from the Primary organization into the Young Men/Young Women Youth organization on January 1 of the current year (e.g. 2024).
Said another way: any child who is 11 years of age on January 1 is, on that date, now a young man or young woman UNLESS their birthday happened to be on January 1 (i.e. the day prior, they were 10 years of age.)
So effectively, you're describing a Youth campingi trip involving both the young men and young women.
2) General Handbook 20.5.5 discusses overnight activities for combined groups of young men and young women.
Notable:
* Parents/guardians must give permission for their youth to attend overnight activities
* Overnight activities involving combined young men & young women require approval from both the Bishop and Stake President
* All the usual youth protection guidelines apply and are outlined in that section (multiple & sufficient YW leaders present for the YW, multiple & sufficient YM leaders for the YM, Priesthood presence, ...)
Based on the info shared (and provided the Bishop and Stake President sign off on the activity), I personally don't see anything in conflict with the policy.
(Keep in mind, I'm not particularly up-to-date on ALL the current youth policies -- 20.5.5 is just the section that seemed applicable as I skimmed the topics)
Of course parents always have the option of not allowing their youth to attend, but it's more fun for the youth if all their friends are present.
The leaders will likely want to chat with the parents individually to get an idea of what their specific concerns are.
(Maybe for parents with multiple youth, Mom wants to attend with her daughter and Dad wants to attend with his son...but if they have younger children at home it may cause a concern for them both to be away at the same time, etc.)