Sienna Miller stunned on the red carpet for “Horizon: An American Saga,” chatting with “Extra’s” Vanessa Borge. She said the experience on the Kevin Costner set was “like kids in a playground” and very “warm and magical.” She also talked about working with Kevin’s son 15-year-old son Hayes, who she said was very professional and really delivered, noting that Kevin was hard on him! Her own daughter Marlowe “chickened out” of a cameo, but Sienna said she could pop up in the third movie of the saga. “Horizon: An American Saga” is in theaters June 28.