On interior doors? Most (all?) interior door locks have a little hole in the knob that you can push a wire into to unlock the door. Interior locks are just for privacy, not security. So if they happen to accidentally lock the door or if they lock themselves into a room and won't open the door, you can easily unlock it.

Yes, interior doors. And yes, most doors have that hole, however I would like to avoid the whole problem if possible.

The only way I know to avoid it is to put on new door knobs without any locking mechanism

There are definitely locks on the market that do what I'm looking for. I've seen them with my own eyes in different places.

I found this online:

Dormitory Lock: Deadlocking latch bolt by levers except when locked by push button in inside lever. Key in outside lever locks or unlocks outside lever and releases button. Closing door releases push button. Inside lever always free.

Source

However, when I click on the link, there are locks that cost $175-$230! I'm looking for a cheap one for $10-$20...

Thanks for your help!

We had a short piece of metal coat hangar that was bent at the top and hung on a nail at the top of the door frame. With 4 kids it came in handy!

Did you ever find a cheap version of this? I am looking for the exact same thing and came across this old post.

Since someone asked for a cheap version, and I am in Canada so don't know if USA has code requiring privacy lock sets for bedroom doors but would recommend buying simple passage door handles, no privacy function. Recently did a repair in a rental house, child locked door and closed it upon leaving, parents btoke door frame/casing forcing it open.

I know this is old but, I found one. Arrow M02

do you mean closet lock?

It is a privacy lock that automatically unlocks when you close the door. I am not sure if they still make them, but there are a lot for sale on ebay.