We have gone through a variety of child locks on our doors to prevent our 3-year old from escaping the house without notice. He's quick. At 18 months, he learned the baby door lock on the handle. We then progressed to a hotel chain lock, which he figured out at about 2 or 2 1/2. That finally broke and we found the GlideLok.
Needless to say, this is the first thing that has worked for a while. Don't buy the plastic imitation products that are out there. Those are flimsy, but the authentic GlideLok is made of metal and stands up to use and abuse.