Another update: Just one baby left now. The other one was not thriving, so I culled him a couple of months ago. I suspected hardware disease, and that it was: he had at some point eaten a 2-inch long thin metal stick, and it had lodged in his gizzard, which had become deformed and more than twice the size it should have been - it was the size and shape of a large baking potato. It must have hurt the poor guy to eat, but I kept hoping he'd feel better.
The surviving "baby" is huge. He's most definitely a gander. Mama goose and baby still get along great and enjoy each other's company, and they didn't seem to miss the other gosling much. I still take them for a walk twice a day, and they both enjoy it. I had hoped he was a she and thus named him, but as it became clear he was a male, his name had to be changed. He now answers to the name Jensen. Well, actually he only answers to the possibility of treats.
He wasn't very safe to walk with at first. He kept walking in front of cars and chasing them when they had passed. That issue is much better now; I've been training him to come get a treat whenever a car gets close - or a bicycle, or a jogger. It works like a charm. Sometimes he even comes running to me as soon as he spots a car before I call him. It helps a lot that mama goose totally ignores most traffic.
He doesn't feel safe around strangers, but so far he just stands tall, almost hisses, and keeps his distance. That behavior is probably prone to change come spring, but I'll take it as it comes. Hopefully I'll still be able to walk with them. Mama goose has let her guard down a lot, almost to normal level. But she does react somewhat to his behavior, especially if he hisses at someone.
Here they are enjoying a pool of rainwater:
P.S.: Is there an English word for what they are doing? All I've been able to come up with is "grubbing", is that correct?