each of my kids have the preschool backpack from pottery barn kids (they have some cute new ones since I purchased them last). It's big enough to fit clothes, wipes, a few diapers and a sippy cup/bottle in the outside holder. I found this much easier than a diaper bag for two since you don't always have both with you, or each of you might have one going to two different places. They are light enough that the three older ones can carry their own, leaving me to just deal with the babies stuff. I also have a large tote bag that I keep in the car so I can throw them all into that if needed.
Re: What diaper bag do you use?