Shampoo and other soaps / detergents should take any urushiol oil out of your hair. Having PI really means that you are having an allergic reaction to contact with urushiol oil. Once you've done a good job of cleaning up, there is little likelihood of reinfection. If the little girl who had a PI rash had properly cleaned up, then its again highly unlikely that you would have been in contact with urushiol oil from her. The rashes themselves are not contagious.
If you are very concerned about having urushiol oil still in your hair, then I would say using Dawn or other similar detergent certainly couldn't hurt and would give you some peace of mind.