Awhile back, I read an article about helping kids to learn to ride. The trick is to take off the pedals (and training wheels), and make sure the seat is low enough that they can firmly stand on the ground. Then let them push the bike along until they get a sense for balancing and steering. Use progressively steeper and longer hills that they can coast down to practice at speed without having to coordinate the pedaling.
It sounded like a good idea, and sure enough it did the trick! We started out inside the garage at the top of our very short, steep driveway. I waited across the alleyway and Alex rode down to me. He repeated this 7 or 8 times. Then I had him start from the back of the garage to build up speed. He did this several more times. Then I had him start to turn when he got to where I was standing in the alley. Finally, he was ready to go over to the "yellow park" where there is a large circular concrete path that goes up, then back down a gentle hill, and is surrounded by lush grass. I brought along his pedals and a wrench, in case he started to really get the hang of it.
After about 3 times around the circle, with me pushing him up the hill and him coasting down the hill and around, he stopped for a break. Then we put the pedals on. He was apprehensive, so I reassured him that they were "just for resting his feet on" while he continued practicing balancing and coasting. But sure enough while I was still pushing him up the hill the first time, he started pedaling and was off on his own before I realized what was happening.
Our neighbor and friend Kaylin, also 6, who has been riding for at least a year now, came to the park with us, and she offered tons of support and encouragement as she rode along with us. Definitely a helpful touch!
The rest of the afternoon, Alex was riding all over the alley and telling everyone who came by that he was riding without training wheels! A very proud moment. He and I rode together back to the yellow park later this afternoon for a "victory lap".
Here's a short video of Alex taking one of his first laps around the yellow park loop:
