When I want a rooster to be friendly to young chicks, I pick a rooster that is fully adult and by himself. I keep a lot of roosters, most penned and most of those without female companions. Such roosters readily adopt chicks as it can be a mechanism to start a harem if adult females are not present. When I say chicks, they are usually at least 3 weeks old. I have had some older cocks adopt chicks only a week old but that only works in very warm weather as during first few days and night the chicks need brooding the cock is slow to provide. This behavior is outside what most think possible with roosters but can be promoted relatively easily. I have even had cockerels do same but results much less consistent. See images here of cock brooding chicks. Further details in thread. https://www.backyardchickens.com/t/882368/what-to-look-for-in-a-broody-rooster I have had similar results where cock is confined in large rabbit hutch for a few days with a large brood. Rooster must undergo what I think are hormonal changes that adjust his behavior. Those changes are induced in part by the chicks coming into contact with roosters breast area. Rooster does not develop brood patch like hen but he still provides considerable warmth.