Q. My 2 year old figured out how to climb out of his crib, so we moved him to a big boy bed. Now he routinely gets out of bed at night and goes to bother his 5 year old sister. Is it cruel to lock him in his room at night?
A. Your child's crib was a safe place. Now that he has graduated from the crib, you need to create a similar safe place for him. His bedroom should become his new "crib"--he can do whatever he wants in his bedroom, but he is not free to roam around the house at all hours of the night. It's not just disruptive--it's not safe.
With that in mind, I suggest making his bedroom completely safe for him to roam around. For example: make sure bookshelves and other heavy furniture (that is tempting to climb) is secured to the wall—it's especially important for those kids who enjoy doing crib acrobatics. Then keep him in his bedroom at night by either placing a child safety handle on the door to his room or mounting a couple of safety gates (one on top of the other) so he can see out of his room but he cannot leave his room. This isn't being cruel or extreme, it's being safe.
When he is old enough for you to trust him walking around your house at night, you can remove safety handle or gates.