I live somewhat near you (Wash. State) and have dealt with mites, but not lice.
Lice live on the chicken and thus the main thrust of treatment should be to treat the chicken. The coop really should be cleaned out and treated too, as there are mites that can live in there and only come out at night, or you might have a few mites on the chickens too and not know it, etc.
So I recommend getting some permethrin spray or ravap liquid (just learned about the ravap myself and haven't tried it) and spraying the coop, tossing all bedding and nest box material. Do this one week later to get the hatching eggs as well, so go easy on the bedding.
I also buy Poultry Protector spray for the coop- you can use it on the chicken too. I have seen it kill ants and stun wasps so it is good stuff. Less toxic for the coop if you want to go that route.
Second thing is to treat all the chickens. I personally use Poultry Dust instead of dipping the chicken or spraying them, as you should blow them dry if giving a bath in winter. That is way too much work.
Here is my method (and yes it is time-consuming, allow one hour for as many chickens as you have, maybe more if you are not used to it).
Go into the coop either in the morning when they are still sleepy and it is just before light, or at night before all the light is gone and they are on the roost already. You will be placing them outside the coop when done to keep track thus need it to be light (or run a light in the coop so that you can see what you are doing and the chickens will run back in the coop when you are done with everyone).
Place something on the ground like a plastic bag, or use a box with a plastic bag liner.
Wear gloves, mask, and long sleeves (take shower afterward).
Take a chicken off the roost and place them on the ground (a table makes this so much easier).
Turn the chicken upside down, holding onto the feet with one hand, and use a sock tied off at the top with Poultry Dust (permethrin) inside it as a powder puff. Shake that powder all over the chickens' vent area, under the wings, and abdomen. Turn chicken over and dust back and neck. Don't do the face.
Let chicken go, get next chicken. If you don't get them when they are ON the roost you have to catch them inside the coop which makes the job last three times as long. I once did 105 or so chickens and was almost in tears as I couldn't catch them very well.
Don't put insecticide in the nest boxes. DO dust the little chicks but use your fingers to gently work the dust into the feathers and don't use very much. They will be dusted by being under mom but lice can destroy a flock's health and thus they need to be treated. I wouldn't dust itty bitties but at 3 weeks they are probably spending a lot of time not being underneath mom and I wouldn't trust that they would get treated by mom's feathers at that age.
If you set up a dust box with sand, DE, dirt, woodstove ashes, and some Poultry Dust in there (make it so that it won't catch on fire or the coop on fire with the ashes), they will dust themselves regularly and it will help prevent infestations.
I have to treat my flock every 4 months to keep away mites, I have found. Then repeat in 7 days until bugs gone. If I don't toss all shavings in nest boxes I have to start over.
The life cycle of lice is closer to 2 weeks, which is why some recommend treating again in 2 weeks. I say every week until gone, since you can kill them off before they lay more eggs. I read that somewhere.
Sevin dust is VERY effective but is not labelled for chickens since 2009. So it is illegal to use on poultry, in other words. There are a lot of websites that still recommend it but EPA is very strict and thus the dust for sale now is permethrin at the feed store. The sevin is only in the garden section.
DE is only a preventative (for mites I can attest) and thus I'd use it when you are free and clear.
Unrefined coconut oil I have read will dissolve the nits after the bugs have been killed off and you for cosmetic reasons want those egg casings off your girls. It takes awhile but they apparently disappear with the warmed oil.