Seeking to repeat last night's success, I had Kai on the potty for repeated attempts before outfitting him in Diego underwear. Not five minutes later did I spy a spreading dampness on his pants.
I cleaned him up and tried again. Time passed happily. Right before we got him ready for bed, another special surprise prompted us to give him his first semi-traumatic shower before diaper and pyjamas.
Now we're thinking it might be a while before we try him in undies again.