Comparative form of childly; more like a child or more childish in nature.
From 'childly' (archaic adjective meaning child-like or befitting a child) + '-er' (comparative suffix). This represents an older form of expressing degrees of childishness that has largely been replaced by 'more childish.'
English used to regularly add '-er' and '-est' to adjectives to make comparatives, even awkward ones—'childlier' sounds odd to modern ears because we'd say 'more child-like,' showing how different word-formation rules have become preferred over time.
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