When trying a new type of food with your peacocks, it is safer to ask questions than assume that the new food is harmless.
Peacocks can safely eat grapes. Since they are omnivores, peafowls have a digestive system that can tolerate both plant and animal matter.
Their typical diet in the wild includes fruits, insects, small animals, vegetables, and seeds. And of the many fruits peacocks get to choose from, grapes are one of the favorites.
Peafowls can eat grape seeds. Some sources believe that grapes that come with seeds are a choking hazard. So, you may opt for seedless grapes for your peacocks...