Drew Barrymore likes to think of herself as a “walking cautionary tale” for child stars. The 49-year-old actress was just seven years old when she found fame starring in ‘ET: The Extra Terrestrial’ but endured a string of personal problems in the years that followed and hopes that her stint in a rehabilitation clinic at the age of 13 will serve as a conversation starter about “boundaries” for children in Hollywood. She told People: “When I look at it now, I don’t see sadness or tragedy. I love that I was a walking cautionary tale because then when we talk about how we raise our kids in a world…