Bag for My Baby and My Son
All the camera bags/backpacks with space for additional stuff that I've come across aren't spacious enough for baby stuff. My advice would be to use a regular backpack (or shoulder bag) with a removable padded camera insert.
Crumpler make one called the Crumpler Haven
Other brands have similar products and cheaper unbranded inserts are available on eBay.
If you have too much money burning a hole in your wallet, Billingham make a range of camera bags called the 'Hadley' range where the padded insert is removable. This padded insert is used in the Billingham bag when you're travelling alone and then taken out and put in a family-sized bag for family outings. They also sell the insert separately.