We have two bantam broody girls that hatched 7 chicks about 4 weeks ago. The mamas are raising the chicks together in the chicken coop with the other chickens. The mamas are in a rabbit cage with nice soft bedding, while the big chickens all roost above them - then they all free range during the day. The mamas don't discriminate on which chicks they care for and neither do the chicks - I have watched them go between mamas without any issues at all.
6 of the babies are bantam - OEGB/Cochin mix, but one is a white leghorn/cochin mix. At 4 weeks old, s/he is almost the same size as her mama. It's been pretty cold at night (40's) and when my daughter checked on everyone last night she said that all the babies were under their mamas except for the white chick - "who didn't fit!" She really wanted to put the heat lamp out in the coop.
They have most of their feathers...and it seems that if the chick was truly cold s/he would have squeezed under one of the two mamas. What do you think? Should I add a heat lamp?