why does she call him baby reindeer

As the story goes, when the reindeer were just young calves, they were often called "baby reindeer" by their parents and older siblings. It was a term of endearment and affection, meant to show that they were still young and needed extra care and attention. As the years went by, the name stuck and even now that they are all fully grown reindeer, they still have the nickname "baby reindeer." It's a reminder of their beginnings and the love and support they received from their family.