I had 3 kids under 3 and I only ever used the little bag the hospital sent home with me. I was able to fit everything I needed for a newborn and 2 toddlers (13 months and 28 months) for a day trip in that one bag. We also kept extra diapers and wipes in the car for each child so I could easily repack the bag if needed.
I was able to get 2-5 diapers for each kid (more for the newborn), a small pack of wipes, 2 bottles plus a bottle of water for mixing bottles, formula in small container, 2 changes of clothes for each kid, snacks for the older kids, extra pacifiers, receiving blankets for spitting up, a few toys for the older kids, band-aids and tylenol for owies, cups for the older kids, sometimes books for the older kids, paper & crayons and more.
Last time I only had 1 baby and there was so much room in the bag I didn't know what to do with it.
at 5:13 PM on Jun. 22, 2010