I beleive the temp was “sub-freezing”, not “sub-zero”. Still, one would not take one’s young sons camping at midnight under such conditions.
This is the history for Tooele airport the morning of Dec 6, 2009. http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=KUTLAKEP2&month=12&day=6&year=2009
You can see that the low was 5.5. Simpson Springs generally records temperatures lower than Tooele. Whatever the temperature - whether 5.5 or -5.5 it was too cold to imagine camping with young children.