We have a 5 year old who has opened the windows and pushed the screen of right after a snow storm and went out the window in nothing but a diaper. He took a blanket of course. He can get out of the house in a flash. We put screws in the window frames to keep the windows closed. We had to put so many safety things in. I don't want him to be trapped in the house in any emergency so I make sure that all my neighbors know he can't open his windows. If they see something going on they know they will have to break out the windows. The same thing like when you look at a car window. They often have stickers that say that door has child proof locks. That way if the child is trapped in a car the person trying to save them knows they really can't open the door from the inside.
I was at the point of sleeping in the living room to make sure he didn't sneak out the front door.
I finally say hubby down and we talked about options. He suggested we get a chain lock for the front door. I told him that the little guy only had to get something to climb on and he could slide it sideways and get out.
We decided to put the chain lock up and down with the little knob being able to fit in the hole then slide down automatically. That way it would be a little harder for little fingers. Then we added a key lock to the top. Both hubby and I carry the key to this little lock on our body all the time. If push came to shove my door is not very strong, it's a mobile home door, it is hollow. So if one of us needed the door opened in an emergency and the key was not on us we could easily break the slide part even when the lock and chain are on it.
I added a post to show what kind of lock I am talking about. I sometimes don't describe what I am talking about well.
http://www.homedepot.com/Tools-Hardware-Hardware-Fastener...
Again, we put the part the little knob slides into to lock facing up and down, not sideways like this picture. Gravity made it slide down to where it was locked.