I have a bag for the babies as well. It is so much easier to have everything in on place. Whenever we go out anywhere, all I have to say to hubby is "get the doggie bag" and we don't have to scramble all over the place for stuff, it's already packed other than food and water.
I keep: bowls, utensils, blankies, towel, nutrical, treats bully sticks, baby wipes, pads, extra leash(es) toys, extra clothes and paper/pen in there.