1.
pee, urine
Childish word for urine or the act of urinating. Universally understood and commonly used with children, but also used casually by adults.
Register: Childish but socially acceptable. Commonly used when speaking to or about children, and casually among adults in informal settings.
Common expressions
Alternatives: Adult words are 小便 (blunt, somewhat crude) and 尿 (medical). The polite way to excuse yourself is お手洗いに行く (to go to the restroom).