You can also try mixing a small bit of your chick starter/grower feed into the yogurt and make a warm mash. Make sure to take it up after awhile if they don't eat it so it doesn't spoil and make them sick. You can do several small bits out of one batch by keeping the rest in refrigerator. I'd also cut the grapes up in it too until they get used to the texture. Mine only eat yogurt mixed into their feed or in oatmeal, otherwise it's a yogurt "food fight" everywhere in the coop as they try to shake it off their beaks!
Hope this helps