When trying out the baby seat on my wife's e-bike, it was really easy to fit and remove because her saddle flips up to access the battery. It's a real pain threading the baby seat over the top of my pannier(s) and under the saddle to fit it on my bike.
FWIW the baby seat is a hamax siesta, seat tube mounted, and slightly fiddly to fit even with rack but no panniers. I can't fit the seat then the panniers - there's no access to fasten the panniers.
So is there a flip-up seat post available to fit to a normal bike? Google finds nothing relevant, only ancient forum threads.
Here it is flipped up:
And in riding position:
Detail of the mechanism:
As you can see the standard QR height adjustment is unaffected, and the saddle rails are typical.
I've found out a bit more - apparently there's quite a bit of extra weight (looks like it, plenty of steel in there) and older versions had a tendency to work loose around the hinge.