I have a 5 yr old, and well she isn't the best at listening when there are chicks or ducks brooding in the house, so the extra room has a simple 2$ lock n latch lock at the very top, where an adult can reach but a kid can't. Go to lowes or home depot or any hardware store and buy one. All you have to do is pre-drill you a little spot in the door and door frame, put it about 6" from the top of the door, and this way you can be sure any kids can not reach it.
http://www.arbreptiles.com/cages/3x2_latch.jpg
http://www.arbreptiles.com/cages/3x2_latch.jpg