Hello and
That is indeed a problem. Not many chemicals safe enough to use on hatchlings, none that I know of.
Has she ever hatched chicks before? As in, do you think she'd be okay with a certain degree of interference? If you've ever had to remove her from the nest before and she's gone back, she should be okay with some interference. If she's an unproven mother that's a big risk, since you won't have any guarantees of how she'll react.
If you think she'll go back to the nest you can try dusting her off, caging her while you change all nest materials, and putting her back onto them. This should work if she's bonded to them vocally, i.e. is responding to their peeps. It may pay to wait for night time, do all the dusting and nest material changing, and use a torch to show her the eggs and get her to sit again. Some hens are easier managed at night time.
Identifying what type of mite they are will help too but at this stage getting rid of most of the population is obviously the priority. Your other chooks will almost certainly need doing too, if not now then in the immediate future. You won't be able to get rid of all of the mites on the broody both instantly and safely I expect, since I don't know of any mite treatments safe for chicks, but I use natural methods anyway so don't know that much about chemical ones to begin with. I may be wrong and someone may know chick-safe mite treatments.
If she's eating at all, I'd give her something like hardboiled egg mashed up with a bit of yoghurt and three or four cloves of finely minced garlic mixed in, or some such concoction she shouldn't be able to resist that contains lots of raw garlic. I recently had my chooks catch some very nasty mites off the wild birds because I wasn't giving them their usual garlic maintenance dose, so I got them garlic and dosed their feed (2 cloves per bird average but you can go a fair bit over) and within a few hours the mites abandoned them. It gets through their systems very quickly and repels the mites very effectively, sickens them and rapidly dropped the mite population by 95%, but to kill the survivors it will take a sustained dose for a while.
Obviously, with newly hatched chicks not eating for 48 hours on average, this won't help them though. My hens did hatch chicks just fine despite the mites, but it is a risk of course.
If you're not game to interfere before any hatch and think the level of mites might not be too high, then as soon as one chick hatches and she bonds to it, you should hopefully be safe to move her onto fresh nesting materials and dust her inbetween the move. Many hens are much easier to move once they have shifted their bond from nest/eggs to a hatched chick, obviously.
It's your call, depending on how severe the infestation is. They can kill chooks quickly.
Best wishes and good luck.
