After taking human form, getting married, and having a child of his own, the ancient child eating (though newly reformed) Boogeyman must protect his oblivious human family when he suspects a new monster may lurking under the bed of his son….planning to out the horrible secret of his past life.

Good concept by the way. Has space to breath.

I didn't get the "becoming human", taking form for me is like wearing a mask, you can change back. I think the "reformed" statement isn't needed. To become human I assumed he had somehow changed his ways. The last sentence nearly reduces tension. Just having a boogeyman under the bed is bad enough, we all know what that can mean.

When a rehabilitated Boogeyman turned loving family man discovers a Boogeyman living under his son's bed, he must protect him from the kind of creature he once was.

That his family is unaware is part of the story, but I don't think it is necessary for the logline. The focal point of the story is not that they don't know about his past, but that, ironically, he must protect his family from the kind of creature he used to be.