Apologies if this is inappropriate here, but can anyone point me in the
direction of any online resource for making a simple morse code kit for
use with children?

What I have in mind is a pair of keys separated by 20 or 30 yards of
bell wire or similar, with a buzzer or lamp at each end, activated by
the distant key, powered by a battery. Any help gratefully received,
including a source of cheap, basic morse keys. Thanks very much.