I also second the use of nix as a mite treatment as I've used it in the past on very young corn snakes without any noticable side effects. The dilution I was recommended and that I used with great success is one Nix bottle (59ml/2oz) to 1 gallon of water. Clean and disinfect the terrarium and everything in it then spray it down with the solution and also spray your snakes well, head and all.
I've never seen any mites after a first treatment but have always done a 3 treatment 1/week deal to get rid of mites and it's worked really great for me. I was really scared of using anything else because some of my snake were very young and tiny.
Keeper of 5 snakes, leopard geckos, 1 green iguana, 20+ tarantulas, 2 dogs & a bunch of rats!