I've broken two child proof caps on medium bottles. I guess I pushed too hard downwards while turning, and "crack!" After that it was round and round, but it was never coming off. So out came various tools and I cut the top off them. Fortunately I had Two W&N old standard caps and they still fit even though it had been on a child proof top. Obviously the child proof part is only in the cap. Now one was a Old Holland bottle but the neck size and thread was the same, so naturally I reasoned perhaps this size and thread is fairly universal.
There must be loads of caps that can fulfill this function and I found one immediately. Here in New Zealand most wine bottles are now sealed with a screw cap instead of a cork. Turns out to be a perfect fit and most have that non-leaching silvered inner seal as the medium bottles do. A gentle clean with a moist tissue, dry and then perfect to go.
Most child proof caps don't give me much trouble, but it still so much easier not to have to put down a brush and just give a bottle top a simple light twist.
After two weeks waiting to see if there was some unexpected reaction with a liquin medium and finding none. I've now thrown all the child proofs out and replaced them with wine caps. I didn't know how good that would feel.
Please note we have no small children in residence.