New Names

My children have given themselves new names. I am not kidding. They are calling each other - and responding to each other when using the names as easily as if they were their birth names - "Lily" and "Sam". Should I be concerned? It is only when they are playing pretend, such as playing house, or zoo, or (my personal favorite and most quiet imaginary play) Library. Right now, though, they are playing construction and "Sam" just said "Lily can you please pass me the hammer? I need to fix this table". Cute names, though, right?!