Are my families gas burning fireplaces dangerous?

I am terrified of carbon monoxide and within the past three years my family has put two natural gas burning fireplaces in my house. they are both un vented (they do not have venting going outside). our house is average size and usually we burn them pretty much burn them 24/7.they are the only source of heat in our house all winter long. we have CO detectors by both of them but now that's its getting cold again we have them on again and i always wonder if the CO levels are low enough for the detectors to not pick them up but still dangerous giving the fact that we have two 3 year olds living in this house??? is there anything we should do to make sure they are safe??? please help!