I’m doing a rear seat which I drop off at the daycare. (Not a problem for them, parents also drop off car seats so spouses can pick up). It’s a Blackburn Copilot which latches to an included standard style heavy duty rack. I’m using a backpack but if I needed more space I’d get a frame bag. Panniers would go on the rack but not with the kid seat in place (that’s where his feet go) and a front rack doesn’t work with my frame.

I previously had the kid seat on a different bike that didn't work too well, it had a short rear end and high, forward dropout holes and a very upright position, all of which put kiddo too close to me to use a backpack. So I used a cheap Walmart Bell handlebar bag that hangs from a plastic hoop. In spite of its price point and its looks, it worked great.