I think what a lot of people miss about starting kids on training wheels is that it is not easy to spin the pedals going essentially 0mph. On bikes without training wheels, one must be moving already to begin pedaling or have the pedals in a position to actually push after which one has momentum and can keep pedaling. It is only a bike with training wheels that one can be stationary and have the pedals at the top and bottom of their arcs. Once they are stopped at that position with no forward momentum it is not going to be easy to move them forward. Backwards is easier thanks to the play in the coaster brake, though.

So my suggestion, like many others, is to teach the kid to balance first. With that skill, they can get moving on the bike first then begin to pedal.

If you need some motivation, here is my son at age 2 on his Strider (pedaling at age 3):