拉尿 (lāniào) · to urinate, to take a leak

拉尿 is informal/crude, parallel in register to 拉屎. You'll also hear 撒尿 with the same meaning - slightly more common in speech. Neutral/polite equivalents: 小便 ('small convenience') or simply 上厕所. With small children parents often say 嘘嘘 as baby talk.