I've got three that hatched mid-May and mum has well and truly abandoned them. I think because they're together, they don't seem fussed. When I brought them into the flock a few weeks ago, I brought them in within a run in the main run to begin with for a few days. Then opened it up and let them go in and out. Strangely the chicks have stayed in the separate house whereas mum has returned to the main chicken house at night. Even more strangely, I've since introduced a batch of ducklings that were a week younger so I went through the same process - including separating them out in the house/run. The chicks roosted on top of the run instead. When I tried to put them in the main chicken house, one came out in the morning with a neck injury. So then I let them go they want - result is that the three young(ish) chicks and the ducklings go in together.
Meantime I've a lone duckling that hatched mid-April and is completely attached to his broody mum - including sitting next to her as she lays. She's recently gone broody again (in the duck house!) so he's normally close at hand. No idea how I'm going to break that tie.
My daughter has a book 'There's a cow in the cabbage patch' - all about how all the animals are in the wrong place. I've got a duck in the chicken house (unless she's being broody in the duck house) and three chicks in the duck house. All completely confused. No idea really about whether to separate or not! I just do whatever's easiest...
H