Just call them Grandma Demi and Grandpa Bruce! Demi Moore and Bruce Willis’ eldest daughter, Rumer Willis, 34, announced on Dec. 20 that she’s expecting her first child. The actress and singer’s pregnancy news comes just five weeks after she and boyfriend Derek Richard Thomas, 28, became Instagram official. (The couple confirmed their romance on Nov. 15 with a series of PDA-filled photos taken out in the woods.) Well-wishes for the mom-to-be quickly poured in from celebs. “Congrats, beauty!” wrote pal Brittany Snow, while Aaron Paul chimed in, “I’m so happy for you, my friend. Such beautiful news.” Meanwhile, mom Demi — who, along with her dog, Pilaf, and other daughters, Scout and Tallulah, recently accompanied Rumer on a doctor’s appointment (pictured above) — quipped on Instagram, “Entering my hot…