Hmm... My first thought doesn't seem to work, but I'll share it anyway...
Father and daughter go first and take 5 minutes.
Father carries torch back and takes 1 minute.
Father and son go and take 4 minutes.
Father carries torch back and takes 1 minute.
Father and mother go and take 2 minutes.
That's a total of 13 minutes, so there must be some trick to it that I'm missing. I'll have to give it some more thought... (I hope the answer doesn't involve the father carrying his children or putting out the torch and overcoming his fear of the dark or something like that)
EDIT: Okay, I got it...
Father and mother take 2 minutes.
Father carries torch back and takes 1 minute.
Son and daughter take 5 minutes.
Mother carries torch back and takes 2 minutes.
Father and mother take 2 minutes.
Total = 12 minutes!
