A young man, who is a stranger to OP (and presumably also a stranger to her daughter) and who is volunteering at girl guides, touched the OP's daughter's shoulder.
He didn't say, 'hello, I'm Dave, a new volunteer. We're all playing tig. Would you like to play tig? Then you''re it', and briefly touch. He just dived straight in.
I personally would feel like Dave had breached a boundary. Probably innocently, but it would still feel like a breach to me.
And much like the OP, I too would feel concerned by this.
The OP hasn't done anything wrong in asking Brown Owl if Dave is CRB checked, and who is responsible for supervising his placement.
That's hopefully what all the Mums and Dads dropping off their kids have said. You know, just in case.
We all know how grooming works, don't we? That it's not just the children who get abused are groomed, but the adults too. The parents, the managers, the organisations these people work in. How these people enter the circle of trust, how everyone makes assumptions about how 'safe' they are. How nice they are. How their Mum or Dad or Uncle John is such a well-respected team member, how much they all do for charity. How any of their boundary breaches are minimised and ignored, how those boundary breaches progress, to touches, to tickles, to kisses, to alone-time, and to abuse.
I'm not saying that is what Dave is doing, of course. Dave could be a completely innocent chap and really want to be a youth worker and this is the only placement he's able to get. Maybe his Mum or his Gran was a Brown Owl. Maybe he doesn't really want to be around little girls, maybe he'd rather be at home, playing on his x-box, but how do we know? Dave could be worming his way into the church and it's associated groups for nefarious reasons and he wouldn't be the first to do this, would he?
It would probably benefit Dave to have some safeguarding training, wouldn't it? A class in how to keep himself, and others, as safe as possible.