There is no question that the ability to have a successful outing with multiple children, depends largely upon the the content of your bag.
Since I have an obsession with bags, I own quite a few. Typically, I carry a smaller sized backpack whenever we go out.
I find that a backpack is much easier to transport on our outings than an over-the-shoulder bag. Although, I do have a large over-the-shoulder bag that I keep fully stocked with clothes, jackets and diapers in the car.
My backpack, which I have with me at all times, consists of only two compartments: a large compartment and a smaller compartment. It also has two small mesh pockets on either side.
The large compartment holds our foldable potty seat for use in public restrooms, an assortment of diaper sizes (in a ziploc bag to keep them dry, in the off chance my water bottle sprung a leak) a package of diaper wipes and a small, light-weight receiving blanket.
Now that the kids are a little older, I don't bring along a full change of clothes for each of them in my backpack. Instead, I have a full change of clothes - pants, shirt, underwear, socks - for one 3-year old, and a full change of clothes for the baby.
In the small pocket up front, I have a sunscreen stick and a can of spray on sunscreen (within a ziploc bag in case they leak), a comb, a duck bag-dispenser (full of small trash bags - great for dirty diapers, wet or dirty clothes, or trash), and a small tube of Aquaphor - which I personally think is a magic ointment.
I also have infant Tylenol and a small first aid kit. Within the first aid kit I have band-aids, tweezers, scissors, Neosporin, a snake bite kit (because we live in southern California and you never know), nail clippers, gauze pads, paper tape, dental floss, Tylenol (for me) and a reflective security blanket.
Why a reflective security blanket ... you ask?
Because it takes up a tiny amount of space and if I ever got stranded on my way to Target, I could use it as a blanket to keep us warm (or cool), or as shelter to keep us protected from the elements. I could also use it to collect precipitation so we don't dehydrate, and because it is reflective, it can be spotted by search parties looking for us.
I like to be prepared.
I also have with me a small packet of crackers and a bottle of water, because it's a good idea to have a little something to tide you (more appropriately, your children) over as necessary until you are rescued. Or, your table is ready at the California Pizza Kitchen.
In the mesh pockets on the side, I have a small spray bottle that I keep filled with water. The spray bottle works great to do a quick rinse on dirty hands to remove gross contamination before I use a wipe.
I also use it to spritz-clean our portable potty chair, rinse off pacifiers that fell to the ground - or squirt the kids to keep them from falling asleep in the car, particularly if I want for them to take a nap when we get home.
The best use I have found for this spray bottle is to distract a child who is in the midst of a temper tantrum. I'll ask "Who wants to spray the plants, grass, car, shopping cart?" and it always stops them in their tracks.
Clipped on the back of my bag are several carabiners - which I use for anything and everything. My wallet slips inside my backpack as does my small camera and cell phone. (It took a month of lugging around the Nikon digital SLR, before I rushed out and bought a Canon PowerShot SD850 IS for my "everyday" pictures).
It's not a lot of stuff and the bag is quite compact, but I have used every single thing that I have in it, with the exception of the snake bite kit and reflective blanket. (Knock wood.)
This is the content of my "everyday" bag and I never have it packed so full of stuff that I can't get it closed. There is ample room on top to throw in sweatshirts for the kids and hats. If I need more than that (i.e. for a day at the beach), I have all of these supplies - but just move up a bag size.
Whenever we come back from an outing, I will restock whatever items were used, and store it near the front door, so the next time we are set to leave, I can just grab it and go.
I wish I could pack the kitchen sink - which would be useful particularly if I wanted to bathe the kids while we were out - but that little spray bottle is the next best thing.
What's in your bag that you can't live without?