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