My six year old is autistic and after a couple of rides, she fell in love with cycling. We put her on an Adam's trail a bike and she will ride as far as we can peddle. The motion of the bike really does something for her and she just enjoys it so much and begs to ride all the time.
Her balance is really really bad and she is a very long way from being able to ride on her own. We do have a run bike (also called a balance bike) and she likes that, but is isn't something that she could do and keep up with us. I am hoping that using the run bike will improve her balance to the point that she will ultimately be able to ride on her own, but that is honestly years away.