I let my broodies raise their chicks like you're talking about and I have given the whole flock 'Flock Raiser'. It's bad for the chicks to have layer mash, and it's bad for the layers to eat medicated starter. Flock raiser is a suitable feed for them all.
IF you want the chicks to have access to the starter too, which is a good thing IMO, in addition to feeding the whole flock the flock raiser for a while (mine have oyster shell all the time anyway, even with layer mash) you can build a simple little "shed" that the young ones can get under/into, but that the hens can't get in. Here's one that a BYC member built for her batch of chicks in the last post on this thread here https://www.backyardchickens.com/forum/viewtopic.php?pid=4362166#p4362166
IF you want the chicks to have access to the starter too, which is a good thing IMO, in addition to feeding the whole flock the flock raiser for a while (mine have oyster shell all the time anyway, even with layer mash) you can build a simple little "shed" that the young ones can get under/into, but that the hens can't get in. Here's one that a BYC member built for her batch of chicks in the last post on this thread here https://www.backyardchickens.com/forum/viewtopic.php?pid=4362166#p4362166
