I'm Totatlly Bumfuzzled, What Is Going On?

Try bringing both parent and child out to where the bike is, make sure there is plenty of space around, then let the parent be the one to initiate the activity. That usually works for me when it bugs out. 

And if nothing work and the parents really just don't want the kid to learn how to ride a bike, kids can teach themselves so no need for the milestone to go unfulfilled.