The chicks are going to be living with lice if you don't treat the broody hens now. I don't like to dust new chicks (don't recommend it unless really necessary). So I'd treat those broodies now and repeat every week until gone. If the chicks hatch I'd still treat the broody if it was me (not the chicks).
Clean out all bedding and nest box material with every treatment. You can put the dust everywhere (they don't recommend putting insecticide dust inside nest boxes though) in the coop or you can buy permethrin spray (dilutable) at the feed store to treat the coop and even birds. I like to put cedar shavings in with my pine shavings in the nest boxes to keep bugs out.
I too use an old sock like hdowden, tied off at the top for the dust. I keep a cardboard box lined with a large plastic garbage bag, and place the chicken in the box (shallow box) on her back, holding onto both feet.
I dust under each wing, the vent area, and everywhere except the face. Then I go get the next chicken. I wear a mask and gloves, and take a shower afterward (long sleeves are good too).
It takes me about half a can to treat my 26 large fowl, as I am very generous with the dust as I shake the sock around their feathers. It works very well and I have to do it every 4 months to keep mites away- I haven't had to deal with lice. For the nits after the bugs are dead, I have read on BYC that coconut oil will dissolve them over time with an application of it.
The life cycle of lice I think is two weeks but some sources I have read say that for infestations it is beneficial to treat every week until gone.
I hope all those buggies get gone for you!!!
