Sorry to hear about your pest problem =(
I'm a first time grower, myself. Back when my seedlings were about 1 week old, I was inspecting the leaves/soil/pots and noticed maybe a dozen or so spider mites crawling around in the soil. (If you're not actually looking for them, they would be very hard to spot.) I remembered reading somewhere, that Neem Oil was supposed to be good for pests. I diluted 1/4 of recommended dose into 1 gallon of water, and sprayed on my 1 week old seedlings, the soil, the pots and their plates/saucers. I also made sure to spray just before my lights went off, I didn't want to risk burning any of my seedlings..
My plants are almost 7 Weeks old and I haven't seen any creepy crawlers since. To this day I still use the Neem Oil about once a week, as a preventative measure...
Good luck with your situation!