A lot of people won't mix babies with big girls until they are fully grown. Last time I did it i put them in a 22 weeks but I guess you are in a bit of a bind if your old girl is by herself. One thing I have heard people do is fence of an area with gaps big enough for the littlies to go through...
I have to constantly remind myself to write chicken, not chook, here too. Chook is Aussie for chicken. We're a weird mob you know.
I wouldn't be making any major changes. Your little ones will be on the roosts before long then they'll lose interest in sleeping in the nesting box.