Children collectively; the human species or condition as it exists in childhood.
From 'child' + 'kind' (meaning type, species, or nature). This is a compound meaning 'the child-type' or 'children as a group,' using 'kind' in an archaic sense similar to 'mankind.'
The word 'childkind' mirrors 'mankind'—both use 'kind' to mean 'the human species/group in a particular state'—but 'childkind' is so archaic that most modern speakers wouldn't recognize it, showing how our terminology for life stages has completely transformed.
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