Introducing at night is a good idea to be safe. On the other hand, my bantam (devoted to her nest as you describe Olivia) didn't bat an eye when I stuffed three store-bought chicks under her in broad daylight. She actually helped me tuck them under her and clucked to them like they were her own who had wandered off. Maybe it depends on the hen?
That is so adorable! It is so sweet to see how motherly our hens can be.

