I would think it would be quite normal for a child as creative as yours to do this. Our DD at the same age had people who didn't know us personally convinced that she had an 11 year old sister named Mishemba (my spelling, as I still have no idea how she came up with that name) and could tell them all about her, what she did at school, who her friends were etc.

Perhaps it would only become an issue if it were still present at the other end of her life. Then they would call it delerium. grin