They can have grapes if they will eat them. Not all chickens eat everything you think they would. I’ve been surprised at what some of my chickens have turned up their nose at. Really surprised. But one of the funniest things I’ve seen adult chickens do is stand under a grape vine and jump up to snatch a grape at the top of their jump. Bouncy, bouncy, bouncy.
I’d supply grit at that age anyway, even if all they are eating is chicken feed. I know they don’t need it if chicken feed is all they eat but it doesn’t hurt to have their digestive system set up in case they snag something you don’t expect. If your grapes have seeds, yes they need grit to grind those up. I’d want grit for the grape skins too.