+ 1 re: feeling the power of my 4 yo son pedaling. In fact, I typically increase 1 - 2 mph when he is pedaling.
There is so much for him to learn riding a trail-a-bike. Not only is he learning to shift, but he learns when to shift. This is especially helpful on uphills, particularly single-track. Incidentally, while offroad he is also learning to lean forward while going uphill (increases rear wheel traction), pull on his handlebars towards his seat (again, increases rear wheel traction), shift to an "easy gear" to help with pedaling, and pay attention to pedal position (a low pedal is likely to get caught on something).