This sparked by a comment on thread "American toilets".
I have always used bathroom and bedroom dooor handles that can lock from inside by a quick twist of the handle and with one exception in this house have standardized on WEISER. These are not the keyed type of lock. Haven't bought any for years. But they used to cost, new, around $15 to $20 Canadian. Also since many parts interchangeable sometime at flea-markets/boot sales etc. pick up spares etc.
The point being that even when 'locked' from inside for privacy etc. one can in an emergency push something through a hole on the middle of the outer door handle and get in to help a child or someone having a heart attack or fall, in the bath tub.
Suitable items are a smallish meat skewer, a three inch nail, a piece of the end of a quickly straightened out metal wire coat hangar.
In fact we keep a section of stiff cut off metal coat hangar above the door of the bathroom for just such an event. It's the same tactic as having a fire extinguisher handy. Easy to reach.