Yes, It's beacuase It's still a child. The hair potion is one of the items you can't use until your an adult. I believe that the only items you can use as a child are the ball, and teddy bear. And maybe the action figure but not to sure.
Thank you! I didn't know that the bear made the girls happy and the action figure makes the boys happy. That must be why when I give the bear to my boy tama it gets mad..