The main thing I learnt with nappy bags is to make sure they are light to begin with, as they are only going to get much heavier as you load them.

Honestly, I found a normal bag to be fine, just with some pockets at the front for phone, keys etc. I kept most of the nappy stuff in a clear removable pocket. So if I was leaving one of the girls with someone I would just hand them the pocket or I would just have to grab it for a nappy change.

How much I carried with me depended on how close the car was. With DD1 I used the pram to get everywhere, so needed to carry most stuff with me. With DD2 the car was always close by, so spare clothes, extra nappies etc were just kept in the car. That way I just needed 1-2 nappies and wipes with me