Green’s Dictionary of Slang

1. (Aus.) a child.

1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 9: He had been thinking of his trouble and strife during the fun part. There was no doubt about it. Her and the billy lids would have to come home.
1993		Age (Melbourne) 10 Apr. 26/3: Billy and his Billy-lid arrived.
1999		G. Seal Lingo 91: Then there are those things that form the basis for still more distinctively Australian rhymes, including: [...] billy lids for kids.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 18/1: billy lid n. 1 a child.
2010		J. Miller Lingo Dict.
2011		IOL News 17 Nov. 🌐 [He] wished Obama a safe retuirn home ‘to your cheese and kisses’ and to the ‘billy lids‘.

2. (N.Z. prison) a fool.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 18/1: billy lid n. 3 an idiot.

3. see lid n. (1); lid n. (2)