Green’s Dictionary of Slang

brierpatch child n.

[the image of a brierpatch as being secluded and thus safe for an illegitimate delivery; note US dial. lap child, the youngest child, still confined to their mother’s lap, a spoiled child; yard child, a child old enough to play in the yard]

(US) an illegitimate child.

[US] in PADS 14.
[US] in DARE.