The jet 12 does have a waist belt, but I doubt you would get a synthetic bag into it.
Okay thanks. I'm going to get compression sack for their kelty's synthetics. My youngest carried a standard school LLBean book back and got his bag in it with just the stuff sack. According to what you say i'll assume the jet 12 is smaller than a standard bookbag and i'll be assuming the jet 18 is closer to the standard size.
I appreciate the help.
My only other idea was to buy a smaller camelback hydration pack (since they are smaller in size) and attach their sleeping bags to that externally.