Hello and welcome to the forum, glad to have you here. What Rebecca and Rick said above is good advice.
Neither soil or cotton balls are of any use and will not help. The soil will make things messy when you mist. To give them water, mist with a regular spray bottle filled with water daily. Like Rebecca said, I'd take the soil out, clean the tank, and line the bottom with paper towel.
Your L1 (code for first instar, how old they are at hatching) nymphs need fruit flies. You need to get them in a hurry, or all will eat each other or die. Local PetCo's sell them (not best choice for quality, but good if you're in a pinch and need them fast). Otherwise Google "Buy Fruit Flies" and you can order them online at many places, like Rick said.
You do have a screen lid on the top of your aquarium for ventilation, right? They need ventilation. Otherwise, and eventually anyways, you'll need to separate the nymphs or they will eat each other... even with plenty of food available. Look in the Housing section for some tips of what people keep their mantids in. After getting some fruit flies, reading through past threads in the forum is one of the best ways to learn about keeping your new little babies. And if you get stuck, you can always ask.
Best of luck to you and your little ones, and again, welcome!