Wow - what a drag. So hard to see... And all we want to do is solve it for them, I know.
In reading responses here, I see a few that tell you not to admit and "reinforce" something that is not real... In my mind, that is the WORSE parenting advice I've ever heard! I am sorry for their children that their parents don't listen to what is important to THEM.
I thought the advice about listening and not talking was the best ever... Kudos to that mom.
What is going on for your child is very real to her. Please do not minimize it or tell her it's "not true". In her little mind, it is. It is very real and very frightening.
So let me just share what worked for me. When my daughter was in this phase, I chose to empower her to "make them leave".
So when she thought there was someone in her room (and, believe it or not, she did have a real visitation - someting I did not believe in - that scared ME to death! As evangelical christians, we handled that in a different fashion - it was a bit of a step up from this situation. That was 3 years ago and we haven't seen her since...)
Anyway - I digress! Your child needs to take her power back. And this will serve her well all her life...
I would have my daughter sit up in bed and command "Monster LEAVE!" I told her it was her room and she was the boss. (Much like training a puppy...) The monster can't resist and has to leave.
At 10, she still goes to sleep with her reading light on. I don't care, as long as she goes to sleep. I go in and turn if off later.
Another thought is to have her watch Monsters Inc. That shows monsters to be more scared of us! And it's funny and fun. Take the fear out of it for her.
Good luck!