What should you do about a 2 year old bike thief. ?

This problem started about 2 weeks ago.  When the 2 year old ran over. Jumped on the 4 year old's bike & rode it to her house standing up on the pedals & parked it in her drive way.  The 4 year old came over & got her bike back.  Well it happened again.  But this time when the 4 year old came over to get her bike. The 2 year old grabbed her by the hair. Tossed her on the ground & kicked her several times forcing her to leave. She now thinks the bike is hers. & the 4 year old's parents are mad at us. I don't see why. The 4 year old is much bigger than the 2 year old. & should be able to fight her own battles.