It's not great for them to eat a lot of grass seed and it's expensive for you. I found it was pointless to seed grass where chickens can get at the seeds. They just ate them all.
I made a cover so that I could seed parts of my lawn safe from chickens. I made a simple wood frame, several of them, only about 4 inches hig ~ you can even use canvas stretchers like they sell at an art supply store ~ and covered the top with leftover hardware cloth or tight weave deer fencing from their run. I place these cloth side up on an area and seed underneath. Then the grass is shaded but gets sun and is easy to water. The seeds and sprouting grass blades are protected from chickens, squirrels, wild birds and foot traffic. It actually grows better under there. Grass patch is established in about a week or so. Moves the frame to the next area and repeat. Works great. The chickens just walk over the covers without disturbing the grass.
My frames are about 2 x 4 feet and 6 inches high. I use them to start seeds for everything everywhere. For grass they need be no more than a inch high (so canvas stretchers or old picture frames will work fine) and they can be any dimension you like that is easy for you to manage.