A child; so called from the Welsh, brat, a child's pinafore; and brat is a contraction of brattach, a cloth, also a standard.
“Every man must repair to the brattach of his tribe.” —Scott.
O Israel! O household of the Lord! O Abraham's brats! O brood of blessed seed!
Gascoigne: De Profundis.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894