help! my 3 week old poult has lice/mites bad.

[pigninnie]

I got 3 blue slates last week and while I was checking them over (the butts on two of them have been wet for 2 days) I notice that there was bugs on one, and quite a few and one of the ones who have a wet but also has a puffy stomache.
they are housed with 2 week old buff orphingtons. I just need to know if they are all old enough to dust with the powder from TSC?

 

[mtngirl35]

I have only given my chickens diatomaceous earth mixed with sand to dust bathe in and the new chicks started using it at about a week old and I've never had any mites. I sprinkle it in the coop too. Any feed store should have it. Make sure its food grade.