I've had the same LE backpack diaper bag for the past 6 years. Since weaning DD2 a year ago, I pretty much always leave it in the car unless I'm dropping DD2 off at church nursery. I did use a little clutch with a couple pull-ups when DD1 was potty training, since she and DD1 were in different rooms at church. I'd toss it in the main diaper bag most of the time and just pull it out to go with DD1 at church. But otherwise, it's always just been easier to have one main bag and know what's in it.
It will be an adjustment for me to start carrying a diaper bag around all the time again when #4 gets here (if #4 ever decides to come out...)!
ETA: I realize I didn't answer the age part of your question, OP. DD2 is about the same age as yours, and if I weren't about to have another one, I do think I totally could get away with sticking a couple things in my purse for her.