In reply to John Kelly:
It's actually a long-established principle: best places to look for info are references for paediatric anaesthesia or sports physiology. Conductive heat loss in children is relatively greater than in adults.
But now I'm sounding like an arse, which wasn't my intention
The point I was making really is that decent insulation under the sleeping bag is even more important in kids, and upgrading a mat can extend the comfort range of the bag by a few degrees.
Unfortunately, there's relatively little work done on kids' bags in terms of lower comfort ratings, for fairly obvious reasons. This means that information comes mainly from parents' reviews, which inevitably tend to be subjective and, as can be seen here, quite variable. I think most manufacturers would be shy about producing really high performance bags for littlies, because there's too much potential for it to be a sh*tstorm. I had a bit of a chat with alpkit a couple of years ago about upgrading the alpkid, but they were quite happy about where they were/are. Frustratingly, they don't seem to have much intention of producing a bag for older kids either.
Hope this isn't taking it too much off topic, or destroying everybody's will to live.
Martin