I wish they showed some more views, but I've used this bag for our past two trips and really like it:
http://www.llbean.com/webapp/wcs/st...ry=4520&cat4=2913&shop_method=pp&feat=4520-tn
I have the larger one. The front pouch zips opens and has slots for cards, a small zippered compartment, and a clip on an elastic string for keys (I used it for a pacifier). The mesh pocket on the side is good for a sippy cup, bottle of water or sunscreen. The other side has two compartments that are really handy for a phone and small bottles like tylenol or hand sanitizer. There is a large pocket on the back that velcros shut that is great for park maps and paper things. The inside has a zipper compartment and a few large mesh pockets. The strap is adjustable and can hang on the back of a stroller. I found it had just the right amount of pockets and compartments: not too many that I was searching through everything but enough to organize.
I also have the ever popular Baggallini Messenger bag and liked using that too. Most days I would pack the LL Bean bag to stay with the stroller (no valubles or anything irreplacable, obviously) and pack the Baggallini to stay with me and take on the rides. The LL Bean bag would be just fine on it's own though.
My kids are 2 and 5, if that helps any.