What's your question? Ask

As a former chicken owner and gardener, I will say that chickens will eat nearly anything they are presented. Food is food and they don't care if it's your pretty flowers.

As for the safety of Cosmos for the hens, I would suggest contacting a vet or someone more specialized on chicken care.

This link you may find interesting.

http://www.fresheggsdaily.com/2012/05/top-ten-flowers-your-chickens-will-love.html

I would suggest keeping hens confined to the coop until plants are a bit taller and bigger in spring. Hens can be less interested in bigger plants then tender seedlings.

If you have an annual Cosmos variety, you can cut them back to the ground now. With perennials, it is best to let them keep at least some of their leaves until the leaves die by themsleves. You can cut them back part way now to improve appearance, and then cut them back all the way when the foliage dies.

Cosmos is an annual in northern USA. It does not self-sow as seeds don't survive the winter. Seeds can be sown outdoors once all risk of frost is past and soil is warming. Cosmos prefers full sun.

When you start to see roots sticking out the sides of the peat pots, it is time to pot them up. Cosmos don't like a lot of handling so it is ideal if they can be hardened off and put in the garden at this time. Do not fertilize cosmos as this results in excessive green growth and fewer flowers. Spacing depends on which cosmos you have as some are bigger than others. They need lots of sun and will grow sideways to get it if needed. If your seed packet says to plant 18" apart in the ground, don't go more than 2 in a 12" diameter container. https://www.gardeningknowhow.com/ornamental/flowers/cosmos/growing-cosmos-in-pots.htm

A sulking plant is common even a week after transplant. Cosmos are known not to like this disturbance which is why some gardens feel there is no advantage to starting them indoors. Keeping them in a cool environment with minimal fertilizer and adequate supplemental lighting will help keep them from outgrowing their 3 inch pots. Tall cosmos, over 4 ft, are not appropriate for containers and are likely to flop. All plants in containers develop deeper roots since lateral spread is contained. A 10-12 inch container for each 3-4 ft. cosmos should be enough.

You may want to have a container that is closer to 8" in depth for Cosmos.

https://www.gardeningknowhow.com/ornamental/flowers/cosmos/growing-cosmos.htm