Hi A. -
Not much new advice here, but I did want to add my two cents. Definitely get a gate for the top of the stairs. We have a wonderful gate that swings like a door and even has a motion sensor light that lights up the walkway/stairs in the dark. You can leave the gate open or shut, depending on when you need it.
As for the monsters.... we have simply told our daughter since she was very little that monsters are our friends and we like them.... She loves Sesame Street and we remind her about Cookie Monster, Telly Monster, Elmo Monster and Zoe Monster. We tell her that there are no such thing as mean monsters... They are all nice!
And, to keep her in her room and SAFE at night, we have a child SAFETY knob lock on her room. We have them on the front and back doors, too. When she was very little (2) we simply shut the door at night. She has a monitor in her room and if she needed us, she just called for us from her bed, or got up and knocked on her door and we went to her immediately. She was never scared, because she knew that we would always come right away when she needed us. (When she was potty training, she had a special potty right in her room to use during the night if she wanted to....)
Now that she's older and fully potty trained, we leave her door open and just make sure that the stair gate is closed. (She can manuver the stairs just fine, now, but we don't want her to be able to go downstairs and wander around, if she is ever up when we are sleeping.)
The door lock works PERFECTLY for getting her to stay in her bed at night, even when we don't close the door. How? We tell her that as long as she stays in bed, the door gets to stay open like a big girl. When she starts doing the, "I need a drink of water.... I need to go potty.... I forgot to kiss you (which she didn't)..." string of excuses to get up, we simply tell her that she can choose to stay in her bed like a big girl, or we will have to close her door until she goes to sleep. (We always open it after she is asleep).
Our daughter would never stay in her toddler bed after she started crawling out of her crib, so this was the safest option for her. As for everyone who says it is cruel to "LOCK" your child in their room... It is alot crueler to leave your little one in an unsafe situation. What if your son woke up and wandered down the stairs and fell or got into something dangerous while you were fast asleep?
Our daughter is happy and well-adjusted and has learned that as long as she is a big girl and only gets up to go to the bathroom or get a drink of water that her door will always be open. It may only take a few days of the door being shut for your son to learn that he must stay in his bed if he wants his door to stay open or it may take longer, but whichever, it is important for his safety. Alot of times, our daughter wants her door to be closed anyway....
Good luck and best wishes!
K.