For demodex treatment you normally build up the dose, starting at a very low dose to see if there are any reactions.
Is it localized or generalized demodex?
Many puppies get localized demodex, and it is immune mediated.
Add a salmon oil to their food for its anti inflammatory benefits and get them on the best food you can afford.
All dogs have demodex mites, only when their immune system is compromised to they get symptomatic with mange. Puppies are more common, as they have huge upheavals in their lives which is stressful as well as a undeveloped immune system.