Green’s Dictionary of Slang

pee-wee v.

[redup. of pee v. (1) but poss. predate of wee n. (1)]

(UK juv.) to urinate.

1888–94

‘Walter’ My Secret Life (1966) II 265: The child [...] was sitting in her lap. In a minute said he, ‘I want to pee-wee’.