Tent heater ideas please

In November I'm going to take my three-year-old son camping with me. We will be in upstate New York were the temperatures can range anywhere from 40? at night all the way down to below zero. If it stays close to 40 he will be okay but anything below that's going to make it very uncomfortable for the little guy that still doesn't understand how a sleeping bag works.

The first thing I was going for was the mister heater little guy heater, But fear of starving my son brain of oxygen has pushed me away from this idea.

This is what we got going on
Were staying in a Oz tent Rv4. I think this tent is to our advantage the thick canvas walls will help hold in the heat.
We will also be sleeping on cots. This is a disadvantage as the cold air will be able to circulate underneath.

I was thinking of the heater buddy as mentioned earlier but I'm afraid of the CO problem. I have an Inverter and was also thinking about hooking it up to run a 120 V little heater but I don't know if that will make the night.

Please let me know what you guys think And thanks for your help in advance.