The movie is about a sister and brother who the brother has an imaginary friend and all of a sudden the brother gets kidnapped and the sister and imaginary friend go under the bed and they find that the boogeyman took him and the girl found out that the boogeyman is actually her imaginary friend when she got older but she forgot about her.

Sounds like Don't Look Under the Bed (1999) (TV) (http://www.imdb.com/title/tt0208101/)