When my DH and I decided to put in a garden and get our first flock of chickens, we decided that we would no longer use chemical fertilizers and weed/bug control. We have an acre lot in a neighborhood in an agricultural district - We have a huge front yard, and very little usable back yard, so the garden and chickens are downhill from the house in the front yard.
We are raising our garden and chickens so we can eat better food, and figured adding the chemicals to the lawn wasn't worth it. Our yard is green - with clover, dandelions, etc. With the garden and chickens at the bottom of our lot, the rain would carry any chemicals down the hill to both areas - we would have no way to prevent it. Luckily, there is no HOA here to tell us we 'have' to maintain a perfect yard.
I am so sorry for your loss. You weren't trying to do anything wrong, and you did try to prevent the chickens from ingesting the chemicals. I am saddened for your daughter's experience, finding a favorite dead.

You do get attached to them.
Edited to add, I did not know that fertilizer would kill them, or that weed killer could after it was supposedly safe to let your children play on that same lawn. Scary thought, that.