Archaic term meaning befitting a child, childlike, or having the qualities of childhood.
From 'child' + '-ly' (suffix meaning 'pertaining to' or 'having qualities of'). This is an archaic formation; modern English prefers 'childlike' to 'childly' for describing positive child qualities.
The formation 'childly' is a fossil in English—we see '-ly' used this way on words like 'lovely,' 'kingly,' and 'scholarly,' but it's mostly archaic now, having been replaced by '-like' compounds for more intuitive adjectives.
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