I have to disagree with just using the safety latch. We stayed at POR 2 years ago at Christmas, when my daughter was 4 1/2 She is on the petite side - she is now 6 1/2, and has only recently reached the 40lb mark so that we can take her out of a child car seat and use a booster seat.
When we first arrived at POR, I made a comment about being concerned that she could get out the door. The trundle bed we requested for her was on the door-side of the room and couldn't be moved. DH insisted that 1) she wasn't tall enough to get the security latch open and 2) she wasn't strong enough to get the heavy door open. She nicely proved him wrong on both counts. And she didn't need a chair for the security latch, she could stretch and do it.
We sat her down and had a nice long talk about not leaving the room without us, which was sufficient for her. I've never been big on baby monitors, so I don't know how that would work, i.e., could she be quiet enough to get out without you knowing. If you don't think a talk is sufficient in this case, I would consider getting a portable door alarm. I used to have one that hung on the handle and was a motion detector - if the door moved, it would go off. Was very comforting when I was traveling on my own, but I can see it working in this case as well.