Escaping

So this doesn't really have to do with pregnancy so I apologize, but if anyone can help me out it would be appreciated. So my fiancé has two kids (3 and 4) from a previous relationship and they are coming to visit us in the summer. We were just thinking we need to child proof our front door because they have a history of escaping. They can figure out the regular child proof door handle things that you buy.. does anyone have any suggestions as to what we could do? Here's a picture of the door, the older one is tall enough to reach the deadbolt to unlock it.