People above suggested some good things, but one reality of church campouts is some kids will want to go who just don't have the right gear, and perhaps can't afford it. Here are some tips:

1) they need good insulation from the ground. An air mattress doesn't really cut it for winter camping, unless you insulate underneath you. Closed-cell foam (like foam rubber) is great. You want at least 2 inches if you can. Sleeping on a cot can also be good, but like the air mattress, you still need insulation under you.

2) If kids don't have a good sleeping bag, have them bring 2 of what they do have, then put one inside the other when they're sleeping. This doubles the crappy insulation to usually being pretty decent.

3) Another trick is to buy some clear plastic ground cloth (like 2ml), and use that to loosely wrap the bag. Make sure to have a breathing space at the top. This traps heat in, and can keep you significantly warmer. This plastic is very cheap at Home Depot.

4) Make sure kids change into fresh underclothing before sleeping. Socks are great to keep your feet warm, but the sweaty, dirty socks they've worn all day are not good for this. They won't keep you as warm as the fresh socks will.

5) Wear a beanie (knit hat) to sleep in.

6) A really nice tip is to take a metal fire bowl, so that you can build a fire without having to dig down too much. Campfires are a must on trips like this, for both fun and warmth. Put rocks, or plywood, or something underneath it so it doesn't melt into the snow too quickly.

7) Make sure everyone has a nice, hot meal to eat. I like to do group meals rather than individual meals on winter campouts to make sure this happens. Dutch oven is a great option, and I can give recipe suggestions if you want them. Also, see here:
https://www.cougarboard.com/board/message.html?id=12479583

8) Someone else said this, but layers are important. Not every kid will have a great coat, and that's kind of a bummer anyway if you ONLY have one coat. Layers are nice to add warmth, but also for removing as the outside temps warm up.

That's all I have for now. Feel free to ask any more questions.