Dr. Mehmet Oz is challenging New York State Attorney General Letitia James over controversial transgender care and sexual reassignment surgery for youths, saying, “Our children are not guinea pigs.” James is threatening legal action against NYU-Langone Medical Center if it doesn’t reinstate its transgender youth health program, which hospital officials announced last month it was scrapping. “Our children are not guinea pigs,” Oz said in the March 10 letter to James, which The Post obtained. “