There is more discussion on sleeping bags on the equipment forum. In a nutshell, sleeping bag ratings assume a pad with an R-factor applicable to the temperature, a bag that fits properly (not too big with a lot of air space), entering the bag properly warmed (so your body heat can warm up the air inside), being properly hydrated and fed (need to produce energy all night), and the bag hood fully cinched up around your face. All of these criteria are rarely met so most people need a bag rated for about 10 degrees cooler than conditions. You are not going to get hypothermia in an under-rated bag but you will not get a good nights sleep. It is always a balance between staying warm enough most nights to sleep with a bag of less weight than you would need for the most severe conditions.
I think a lot of "kids" bags are really poor. If I were taking out a kid, I would give them one of my better bags to use.
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