For your little ladies (we hope) I would try and achieve around 60-75% humidity.
My little seedlings (15 days since sprout) are also in plastic cups, under 9 23 watt 6500k CFL's and the humidity is too low at about 45-50% They are also exhibiting some browning of the under leaves. I am them in little 8 ounce plastic cups for now. I am gonna transplant them tomorrow. Stop by my grow I am putting up some pics.
My point is not to talk about my plants, just to say that little issues like that may spring up. Just try and keep it simple. Whatever the issue is, be it temp, humidity, nutes, or water, it isn't outta control so don't panic.
I think it would be a combination of perhaps a slight over watering and your miracle grow soil. These soils have fairly large concentrations of fertilizer that will be released further with each watering. This soil is not ideal for marijuana, as the time-releasing properties can cause major problems during flowering due to a presence of too much nitrogen. It can even delay flowering and stunt growth.
Now, some do just fine with MG soil and products, but for a beginner, its very easy to burn the plants! I am using FoxFarm Ocean Forrest cut at a ratio of 3:1 with FoxFarm Light Warrior. This soil configuration provides excellent drainage, a variety of plant friendly nutrients like bat guano and earthworm castings, and has been great for soil growers all over!
Keep growing and let us know what happens!