Done quite a bit of tent camping with Boy Scouts...
Anticipate that at least some of these kids will never have been camping before. Thus they will arrive completely unprepared and likely with entirely wrong gear.
Your job is to head off that disaster so their first camping experience is not so bad that it becomes their last camping experience. Start the preparing now and communicate with the parents/kids of what/why/etc.
They will need good sleeping bags truly rated below possible low temperature. Mummy bags are better as they have less volume to keep warm. They can also be closed up around head and shoulders to keep from "Breathing" and losing heat out the top. Sleeping bag liners can be good to gain a few degrees help here.
Others have mentioned ground tarps and insulating pads under sleeping bags and tents. The sleeping bag insulation gets compressed under the body so it does not work so well there thus making the pad even more important. What has not been mentioned is using Reflectix foil faced bubble wrap as a sleeping pad. There are three ways we lose heat and the foil really does reflect the radiated heat. Every try a "space blanket"? You can feel the effect. Plus the added air space of the bubble wrap provides a barrier for conductive heat loss.
It has been mentioned about buddy system... but this is really no joke and multiple bodies inside a tent will keep it warmer than a single body. Fill up every tent with no solos. Fewer big tents are likely better than more little tents.
Sleep wear should be a base layer rather than PJs. Just like going out in the snow. Sleep wear should also include a beanie as the body loses a majority of its heat through the head. Can also add an insulation layer over the base layer.
Moisture and cotton are the enemies. Cotton, especially when wet, serves as a very good heat sink. Forbid it in favor of wool and synthetic fibers. Note that our bodies radiate a lot of moisture so, contrary to what you may think, ventilation in the tent is required. Have you ever gotten wet inside a rain jacket? Same deal here in that sealing up a tent just accumulates radiated moisture and reflects it back. Also kids can be very active, maybe more so with boys, and will generate heat/sweat that clothing must handle. This is why the earlier recommendation of fresh clothes for sleeping: to make sure the clothing is dry when they go to bed.
Since I mentioned it, you may consider space blankets. My experience is these will keep you warmer outdoors. But they will also do a great job of collecting water: sealed barrier, radiated moisture, cold surface == condensed water. I was quite warm for half the night and semi-miserable for the other half the night when my sleeping bag and toes got wet.
The body needs fuel to keep its heat pump running over night. Use this as an excuse to sugar them up before bed. This will make you quite popular as well
I would generally avoid heaters and such in the tents. Too many ways for that to go horribly wrong with fabrics that can melt/burn and not much free space. What you *may* be able to do is put a *sealed* hot water bottle inside the sleeping bag for a bit of a boost. This will help comfort for a while and help going to sleep.