I recently sold my medium Carly on ebay because it would not stay on my shoulder. This bag definately isn't for everyone. I heard the large stays on the shoulder better. I opted for a duffle w/ the long strap so I could wear it cross body (4 kids so I can't be constantly trying to keep a purse on my shoulder).
The duffles are great bags for running around with kids...
We also have a thread about best bags for moms on the go. You're very right about trying to keep up with a bag and kids--can't do both and keep your sanity!