I get most of my bag patterns from these two sites.
http://www.anniescatalog.com/
http://www.e-patternscentral.com/list.html?cat_id=8
I have a tip for your DGDIL, which I got from my DD -- give every child who can walk their own backpack. Each child carries his own stuff, lessening the burden for mom. Those in diapers can carry a small packet of wipes and a couple diapers,, those who are older can carry toys, books, etc. Each child carries his/her own snacks/cup.