No problem with grape seeds - they absolutely love grapes, even those with seeds. However, they might not eat them whole. At least mine never have. When I am tired of making jelly, the hens gather under our grape arbor while I squeeze the pulp out the skins one by one for them. They love that squishy pulp, but don't touch the whole grape.
In winter, store bought grapes are never wasted over here. We chop them out, throw them out to the chickens, and watch the feeding frenzy.
I've given them many different treats, and grapes, seeded or seedless, are one of their favorites.