When our twins were around 2 years old, it became clear that we needed a way to lock the front door (in addition to the deadbolt and door knob, which they could reach). I wanted something cheap, effective, easy to install and discreet (not a huge deadbolt). From a friend's recommendation, I bought this lock. It has been a lifesaver.
Of course, I taught my kids not to leave the house without me. But with a partner in crime (aren't twins fun?), they always found a way to get the door open (working together, using chairs, etc). I needed to be sure that they didn't open the door when I was occupied… you know, with such frivolous things as using the restroom. I was also worried that they might open the door for a stranger.
This lock is perfect for a lot of reasons. First, it is affordable, currently under $3. You can't beat that.
Secondly, you can barely see it. It's hard to even find it in the photo below. It's on the right-hand side, a little more than a foot down from the top of the door. It blends in like a charm (or like a jaguar in the jungle, as Diego would say).
Another great thing about this lock is that it screws into the door jamb. This is great for homeowners and renters alike, because you can remove it with minimal damage. Just fill a couple holes, add some paint if you want to get fancy, and you won't even be able to tell it was there.
The only negative that I have found with this lock is that the screws aren't great. I stripped one when installing it. But, honestly, I didn't even get another screw (aren't you impressed with my follow-through?) and it has held up with the one screw for two years and counting.
This is what is looks like in the unlocked position.
To lock the door, you slide it over to the left.
And then it drops down about a millimeter and is held in place. You just lift it slightly and pivot to the right to unlock the door.
Of course, to really child-proof this, you want to install it high enough that the kids can't reach it (even with a chair).