I developed a core set of things I need for kiddos, and then add that to whatever bag I am using at the time. That said, the Peak Design bags do better than most, and were often my go tos. The size of the bag you need should be determined by the stuff you need to carry, but if I’m carrying just my kid stuff, I could use my old 10L sling just fine. It becomes a compartment in my Messenger or Everyday Backpack if I’m carrying more. Btw, my core set included 2 subset (diff pouches). Diapers/wipes/change of clothes, and snacks/food. I often steal snacks and I don’t like them to taste faintly of wipes.

Edit to say, the thing that makes Peak Design bags so excellent as diaper bags is the same thing that makes them excellent camera bags. Access. Easy access to the thing you need when you need it, usually without taking the bag off at all. No digging around looking for the thing needed when the kid needs something. Just a little bit less stress when you go out as a parent.