Colin Wilson wrote:
I've got a low wall with coping stone on the top, but the friends of
the neighbours' kid keep sitting on it...

I was toying with the idea of drilling a few holes in the top, and
putting pointed railheads in there, but they'd need securing - all I
can think of is resin, but having never used it, just how strong is
the stuff ?

(i.e. would 12mm in epoxy be enough, or should I look to do an inch
minimum ?)

Injected resin is plenty strong enough, assuming the stone is clean & sound.
I think 12mm is rather shallow for a railhead though - go for the full inch.