Hi T.,
I haven't had time to read the other responses, but every magazine article I've read on this says telling your child "It's not there." Doesn't work b/c obviously they are very much believing their imagination right now and and arguing with them just allows them to stregthen there position that yes, there really is a monster in here.
So, what you do is kinda like play along... All of us moms are actors a little, right? Well, put it to good use here: Come in and say 'Really there is a monster, then look under and do something like Ah, your right! he is right there! Hurry quick grab him! He is trying to get away! Then you can do a couple things A.) fake wrestle him to the ground and then open the window and fake kick the monster out the window or B.) come up with something like Do you know what monsters really don't like?? girly stuff... Let's keep a perfume bottle around and each time he comes we sprits him, he'll melt away like the wicked witch.
I hope you get the idea here and just no matter how frustrating and annoying it can be use your imagination to fix his. Also, this may mean you have to get up several times and kick the monster to the curb, but at least he knows A.) mommy is here for me when I need her B.) I can go to her when I am scared about anything. C.)That he is safe in his own home.
Hope this works!! Special blessings from God to you,
Amanda
p.s.
We always say bedtime prayers too. I feel they help to calm the day from business that stirs dreams and fears. Here is my favorite:
Matthew, Mark, Luke and John
Bless the bed that I lay on.
4 corners to my bed.
4 Angels 'round my head.