Talk to them and listen.
All this talk of “what can be done”. I was an abused child. People knew. As an adult my auntie actually said to me “we knew things weren’t great at home but what could we do?”. Not one single adult ever said to me “are you ok?” Or “is everything all right at home”. I honestly believe that if they had, it would have opened the floodgates and I would have told them. I wasn’t brave enough to raise the subject myself but if someone had just given me a little opening, I think that would have been enough.
Yes you can argue well what could be done with the information, will Social Services do anything etc. But if you don’t try. Then you don’t know. If you have concerns about a child, of course speak to other adults about it. But if you get the chance, ask the child themselves. Let them know they can trust you. They need a friend.