I think you should wait untill the chicks can hold their own with the big girls. I have a neighbor who just lets them all in together and hes lost loads of small chicks through being trampled and bullied and pecked at. When My bertha had hers I kept them seperated until the babies were about 14/15 weeks Bertha is back int he big pen too so she keeps a good eye on them. They are about 19 weeks now and she is still sitting on them at night!
It is funny but they are safe and that is the main thing.
All I can say is don;t rush it. Most chickens stop playing mummy around 6 -8 weeks so Bertha is unusual.