Look, you had a point with their behaviour, but then you reprimanded them like children, in front of children - I'd have left at that point, because I would think to myself 'I don't need to be spoken to like this whilst giving up my time for free'. Then you went a step further and posted on what I presume is a group chat the next day, after you already made your point to them previously (she may have answered you back, which you didn't like, but you made your point already!). If I was getting messages like this when going about my day, I'd feel nagged and honestly couldn't be bothered with it. You went too far with sending that message on the group chat I think, hence why today you lost a volunteer, which you say you couldn't afford to lose.
They're not there to just doss about, they are there to help aid the group, but, they're not there to behave as your paid employee either, because they are not. I've dealt with my fair share of lazy volunteers who don't take it seriously, and they absolutely can become more of a hinderance than a help, so I do sympathise, but respect goes both ways. Swinging your bollocks at people in front of kids will not make people respect you.
My advice would have been to have left it, after having the word with them, and see if the behaviour was repeated, before going on about it AGAIN in a group message after you spoke to them.
In fact, I would have waited to speak to the adults privately about it, or, if you really just couldn't wait, just said to the girls to hang on a mo, and ask the adult women 'can I borrow you for a sec, I need your help with something' and then had a quick word, explaining it's hard to manage the behaviour of the girls if the adults are talking, too, and could they please have a think about how it comes across them.
I'm not saying you would have immediately got the result you wanted, but it's often better to broach things gently and respectfully at the start. Particularly if you have been in your own words, looking the other way and allowing this behaviour for 2 whole terms - that's not good leadership. This is what being a leader is about - knowing how to lead people. If you speak to adults like children, they are going to get their backs up, and you're likely going to gain the opposite of the desired result. You need to be able to switch communication styles to fit whom you're talking to.
Yes, they were behaving in not the correct way, and you had every right to deal with it, as ultimately it's not fair on the kids and is having their time disrupted. That being said, this is all being taken far too seriously for what should be a fun group meeting, and it's important to bear in mind that every adult there volunteering their time, not just you, is doing so for the benefit of the kids. That's what it's all about.
You come across very school teacher-esque, and that you're taking this far, far too seriously, and personally, to boot.