if the chain comes off  as the child  is peddeling it , . i would look at the big chain wheel.  thats one were  the pedels are , that has all the teeth  on it , and see if one of them are bent, , put the bike up side down , and pedel it around , slow and see were it comes off , you might be able to tap it back into shape ,,,,,,
                 mark
                
 June 2009