There has been some interesting discussion on this thread. But some facts, because whether or not you agree with the policy/have an opinion on trans inclusivity, it's important to get the facts of the GG policy correct.
Firstly, this is not 'new'. This has been the GG policy for at least the last 5 years, in line with Girl Scouts USA and other WAGGGS members. The policy is now written down, and available on the GG site for parents to see, to aid transparency. No, this does not mean that parents get the right to know if a new member is trans because it's none of their business. You can view the policy, if you find it unacceptable you can choose to withdraw your daughter(s).
Age Grouping - Guiding is available to young members aged 5-25 (they can no longer be young members once they turn 26). Now, the age groups are Rainbows (5-7) Brownies (7-10) and Guides (10-14). As a rule, camping only happens in Guides, with Brownies sometimes camping but usually doing an indoor residential (pack holiday). At camp, tents are created with groups of friends or girls from the same unit (if a mixed unit camp) and usually of around the same age. So you wouldn't have a super-mixed age tent, because of privacy and understanding that 10 year olds and 14 year olds would likely have different needs and interests.
Adult (young) Members - Currently (and I say currently, because there are big changes coming in the next 3 years) Senior Section spans the ages of 14-26. This is a giant age group and again, if camping with a section that had this wide of an age gap within the unit, girls would be split according to age. So you would never get a 17/18/24 year old trans woman sharing a tent with a 14 year old. In the next 3 years, the Senior Section will be split, 14-18, and 18-26 with the older group no longer having meetings and instead taking on leadership roles.
Leaders, gender identity, facilities at camp/residentials - So currently, men and women can be members of guiding (this has been the case for a while) male-identifying members cannot be 'leaders' as they cannot take the promise (a requirement for the leadership qualification). They can be Unit Helpers and can form part of the leadership team, and they can go on residentials and camps and help at sleepovers. Women (cis or trans) can be leaders, and also go to residentials and camps. While at camp/residentials it is usual for leaders to have separate bathroom/tent/shower facilities, or if the showers/toilets are shared to attempt to use them when young members are not present. Toilets are rarely, if ever, 'open plan' so you wouldn't be able to see who was in the cubicle anyway.
So, not trying to get into the ethics of the trans debate, each to their own, but Guiding is inclusive and has been for a very long time, and they have done so on consultation with members (young and adult) so you can contact them, but it likely won't change the policy.