The only line followers are wheeled robots.  They don't put machine vision in biped toys.   It might be an interesting variant on the line following concept to have a biped.  Even better would be a biped running robot.  There aren't any running biped robots, big or small.  They're all quadrupeds.  Not sure why this is.  They obviously have the actuator speed.  The closest things are single leg pogo robots.  They have to jump to move, regardless of speed.