I find it funny that so many people have mentioned diaper bags. With two little ones over the last four years, I have often put my gear in our diaper bag. (Not one of the luxury ones mentioned above, just a cheapo plain black messenger bag type.) It is great for protecting gear, because there is loads of padding in there from diapers, bibs, etc. However, you take the risk that sometimes banana goo or milk gets on your gear.
But why hasn't anyone stated the obvious? Such undercover tactics only work when they are authentic. No point to it unless the disguise includes kids, pram, and partner (wife in my case) in the near vicinity. So in other words, when you are carrying all of the gear involved in going mobile with a baby, then better off to put the camera in the diaper bag rather than carry yet another bag for your gear. For the rest of you, i.e. with no kids or with older kids, just be thankful you don't have to carry diapers and related accessories and use whatever nondescript theft proof option that works where you are.
Cheers,
Rob