I have a 10 month old, 3 year old, and almost 6 year old.
I don't carry a "purse", instead I use a tote bag (I have the kind I can put over 1 shoulder so I can have both hands free. It is actually a diaper bag, but looks like a tot bag (even my non-child friends want it). Inside I carry a "wallet-on-a-string". This way, if I'm running in somewhere real quick or just by myself, I can just grab the wallet and throw the string over my shoulder (Target sells great ones). I keep a key fob to my car clipped onto the string (separate from the key I put into the ignition) to lock the door without having to dig into my bag for the keys while juggling kids. Then, to answer your real question, besides my wallet, I keep:
- wipes - I put them in a ziplock freezer bag - it keeps them fresh/moist a lot longer than the little plastic travel containers do and you can easily see when you are almost out
- 1 extra diaper for the baby (3 year old is potty trained, so nothing for him in my bag, although I keep a spare pair of pants in my car)
- empty bottle plus a ziplock baggie of formula for 1 bottle
- snacks - cup of cereal and box of granola bars
- 1 small toy for each (happy meal size)
- pacifier
- and of course my cell phone - programmed with every number I may need