Also, you'd be surprised at how often people /do/ know what they need, but are afraid to ask for it: embarrassment might be a factor, but often it's the /I KNOW I'll totally lose my grip if they say 'no' or ask 'why'/. Or: the thought /It's impossible./

Maybe your group has a "not in bed during the day" or a "TV only after homework" rule. So a girl just wants to curl up and throw a blanket over her head, but can't. Asking might get her permission but it's more likely a genuine 'why' or outright 'no' will be the answer - which she know will make her feel trapped, thus worse. So she doesn't ask.

But if she writes it in her plan, and the staff agrees with the plan and puts it in her file - she CAN ask to lie down and hide from the world. Because she knows it will be allowed.

A while ago in class I needed to leave, but I didn't dare ask. I was only getting worse and worse and I knew that a 'no' or even just a question ('Why?' 'Is that (really) necessary?') or anything but an immediate "yes of course" would push me over the edge (in my case: total flashback, meaning a lot of screaming and thrashing on the floor). So I stayed. Eventually I asked to go to the bathroom, which was allowed, and didn't return - I went to my homeroom teacher instead.

In a vulnerable moment, anything helpful you ask that's denied leaves you feeling worse. So knowing that, you might decide to just not ask.
But knowing you're going to/supposed to receive permission, you likely will ask.

One of my acute panic/over-stimulation thingies is simply /Walk away/ (remove myself from the situation) and it's been SO helpful! Because by allowing me to put that in my plan, the staff gave me permission to walk away when necessary!
And I would have been allowed to, probably, even if it wasn't written down or agreed upon somewhere. But I'm obedient like that: if I'm supposed to sit at the table, I won't generally leave the table.

Also, you're a step-down from secure, you say. It's a real possibility that some of your girls are afraid of (because of threats from parents/staff/authority, stories from groupmates, own experience, having witnessed it happening, or just because they feel like a hopeless case, nutcase, or a bother) having to go to secure (if they are from 'outside') or having to go back to secure (if they were in a secure setting before) if they 'take' (do) or 'demand' (ask for) things that aren't allowed.
'Anteroactive' (proactive?) agreement/permission can help in a huge way.

Also, as for word choice - "May I sit with you?" feels more supportive and less /I'm a bother/ than if someone asks "Do you want-" or "Do you need-". (Although I can't imagine much other situations where that 'may I' would feel both natural and helpful. Only in the "May I join you" sentences I think)