Mother and chicks by the nature of their bond are well integrated. I would be shocked if the mother is the culprit. Are the group being housed separately or with the flock? What size is their enclosure? Are there signs of physical injury on the chick that died or could there be another cause?Bantam has been a good mama to her 5 week old chick, then this morning we found the chick dead and she had also laid an egg. Did she kill the chick because she laid a new egg?