I'm using a 7 year old Kely Super Tioga (4100??) external frame pack I picked up 7 years ago on sale from Campmor for me. In summer conditions, I have enough extra space to put swimming gear in the bottom (sleeping bag) compartment - which my first kid enjoyed last year.

Kelty's external frames (http://www.kelty.com/c-12-external-frame.aspx) no longer seem to have the long pocket on the left side of the pack that I keep my rain gear and trowel in. However, I've found the pockets to be handy, and not just for kids.

Your other option (which I haven't tried yet) is to either build or strip down an external frame, and tie the stuff sacks to the frame. I'll play with this idea more after I get a scale and find out how much this stuff really weighs. I saw some neat ideas in the Make your own Gear portion of the site that I'd love to play with when I have time and have developed some more skills.

Or you can just go to Campmor and pay them their discount rate for one, no further thought required (http://www.campmor.com/outdoor/gear/SubCategory___40000000226_200368422) until you want to shave some pounds off.