Sounds like things are going pretty good so far
I'm sorry to see you lost a chick. Sometimes (like your case it seems) there just isn't a way to prevent it.

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No one can, honestly answer whats going to happen when they get together, You seem to be thinking it through. You will just have to choose a day when you are going to be around so you can watch them for a while when you allow them together. I have not been very lucky putting chicks with adults---even a broody with chicks----probably because I have RIR hens and they never seem to be real friendly with the others. I had lots of pecked or dead chicks and torn up broody hens trying to protect them so ""I"" never mix them anymore. I just built 3 days ago a 6000sqft run so my young chicks can some-what free range away from the adults. Over the last few years I mainly just kept the young ones in pens until they were grown, then I sell the older chickens and the younger/next group takes over.I have a complicated situation I would love advice with.
So I had 4 hens, and had placed an order for 7 chicks. I was planning to brood them in a separate brooder in the coop and then integrate them when they were a month old or so.
While I was waiting for chicks to arrive my golden laced Wyandotte went broody and was not giving up so I moved her into the brooder pen and had her sit on fake eggs for 2 weeks. I gave her two of the chicks (at night) which she adopted easily and is a great mum. They are 3 weeks old now.
She is the head chicken of the bunch and has been leading her chicks out with the other hens and they are doing great.
So I also have the other 4 (I was afraid to give her all of them - she's only 10 months old and had never had chicks before) which I raised in a separate pen in the garage. I have now moved that pen out to the shed so everyone can see/hear each other.
At what point can I introduce them? One of the brooder babies got out while I was cleaning and the mama mostly ignored it, but there was a warning peck. I am not worried about the other hens attacking any of the chicks - they are all afraid of mama and are staying away from all the chicks for now.
Is there a chance the mama hen will accept them now, at 3 weeks? What's the safest way to try to integrate them? The brooder pen is a mesh puppy playpen and mama seems very interested in the chicks that aren't "hers" - she clucks at them and is constantly checking on them.