...and now I can a little bit more. I gave the iBert front-mounted child seat one more try. I still have to ride a little bow-legged, and the saddle has to be too low, but at least it is ridable. And K. enjoys it enough that he puts up with being bundled up and wearing a helmet. He rings the bell and pretends we are a fire engine. It's nice that I can see what he's doing and talk to him and touch him, unlike if he was in the back.
I'm still looking for ways to make it better. I'm short so I have a small bike which means the seat sits back really far, close to me. I can't step off the saddle and over the top tube, I have to stand over the saddle. Our first ride, before I lowered the saddle was a bit harrowing, I could only stop if I put my foot on a curb. The seat also pressing on the shifter for the cogs, making it a little hard to shift.
We've gone out a couple times, not super far, 30-45 minutes, but that's better than nothing. Carrying an extra 25lbs makes for a good workout. I'm glad I have the super low gears I scoffed at.