This is one of the better childproofing kits I have found. It's nice because you can't really lose the key because you can stick it to the fridge.

I used these on some really think cabinets and sometimes it was hard to unlock, once you figured out where the key had to go on the cabinets it was great. What's really nice about these too is that you don't have to take them off if you don't need to use them anymore. They have this little latch you can use to make it inactive.