I do not know how old a child it will stop, but so far it has cat proofed a door. Two other things should be mentioned, (1) it is reversible so it can be installed on either side of a door, (2) the door can be still be opened from the other side. However the weight of the lock and/or the added friction effects the door handle returning to the horizontal, i.e. locked position, so you should check to see that the door is locked after you close the door.

The good: it is a lever knob lock that allows the lever to be turned from the opposite side. It required no adjustment on our levers. It kind of worked at first. Installed on the inside of the master bedroom door it kept the dogs out during the day (German Shepherd bumps the levers UP with his nose, and once installed the levers can only be rotated DOWN on the opposite side), kept dogs in the bedroom at night and out of the kitchen trash, and kids could still get in at night if they needed us.

The bad: big and ugly, as I mentioned, clicks pretty loudly when it automatically re- engages itself when you let go of the door handle (not good if spouse is a soft sleeper), kids (2 and 3) figured out how to operate unlock it within less than a week, and the adhesive installation was not strong enough to withstand repeated attempts by the German Shepherd to bump the lever open. Once he figured out that it was wiggling a little bit, it was all over, and he knew he could open the door at will if he bumped it hard enough. I almost screwed the locking portion to the door, but bought a $8 keyed round knob at Wal Mart instead.

Grandma and grandpa can't figure it out. Grandpa broke one of these off quickly after installing - my son who is 27 months now knows how to open it but it has been a deterrent at least. Our front door had a slightly larger than standard handle (Pella door) but I was able to modify to work. It's not perfect since it's just adhesive but I did not want a permanent modification to our door either. It is hard to go in and out of the house with your hands full - not always easy to operate one handed but you can get used to it.

I first bought the Safety First kit but they did not fit most of my doors . The Kidco kit fit my handle door knobs and doesn't lock out the opposite handle. My kids immediately studied the kit and moved onto the one door that did not have a lock.

I knocked down one star because the clear is ugly. I probably should have gotten white, but now I will just match with the 'clear'

Two clarification points about the product - first, if you have a handle/deadbolt combination system (usually on a plate, popular in apartment buildings and condos), there may not be enough space between the handle and the deadbolt to actually put the top part of the lock (the part that actually locks the handle). Second, it will only work if your lever is straight and doesn't have a bend in at the end, like mine does. Luckily, I was able to drill a hole in the case to make room for that part. The cover itself without the locking part is enough to deter my cat, but that might be an issue for kids.