I've never had mites on the babies before but I think I'd carefully rub ash down under the down and feathers as carefully as possible so as not to get too much into their respiratory system. It only takes a little on a tiny one.
Now yours are with a broody and if you give her a dust bath area she'll have them right in there so they may be able to take care of it themselves. Just put a small amount of the ash into the dirt and see if mom will do the job.
I think that would be my first thing to try before doing anything stronger but watch closely. You don't want mites on the littles for long.
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On the spray, This is a recipe for ADULT CHICKENS.
10 ounces of water
1 ounce of garlic juice
1 teaspoon (total) any combination of these essential oils - bay, cinnamon, clove, coriander, lavender, spearmint, tea tree and/or thyme
Mix in a spray bottle and spray hens bi-weekly as a preventative or every other day for two to three weeks in the case of an infestation. Concentrate around the vent and under the wings.
BUT FOR THE BABIES I WOULD DILUTE IT WITH MORE WATER. Essential oils are very strong.
I ALSO WOULD NOT SPRAY ON THE BABIES. I'd take a little drop of it and put it on them - perhaps near the tail area so it's not right close to their eyes.