hi and welcome. first dont worry, it takes a serious mite problem to kill a bird but they can become a bit anaemic and poorly and the sooner you treat the better. make sure you fully clean and disinfect the coop completely and repeat the clean every few days for a couple of weeks to ensure no wee buggers have been missed. I also dust the walls with diatom and brush them down so it gets right in the cracks. Don't use hay as bedding, straw is better, or sand, and a wee bit of orange peel and lavender buds in the bedding discourages mites but wont harm your birds, they may even like the smell!
and as TC says make sure you really treat the birds themselves well over a couple of repeated treatments. I use diatom and protect their faces with a cloth while applying but I know some bird keepers dont like it as it can cause respiratory problems. Make sure any treatment gets right in all the feathers and skin and around sensitive areas like the vent (mites love a juicy vent) and give them a good check over regularly even after you are sure the mites are gone.
When I got my girls from a rescue they were literally crawling with mites and lice, disgusting! but I treated them for 2 weeks intensively and made sure they had good access to dust baths and they have been happily mite free ever since.
I'd also suggest doing their legs and combs with a little olive oil or coconut oil at the same time as it prevents leg mites getting hold and only adds a few more minutes stress per bird to treat at the same time. Slightly warmed coconut oil mixed with a tiny drop tea tree oil is my magic mix and I brush it on with a pastry brush to ensure it gets in their leg scales without hurting them.