My sister is 5 years old, my Dad told me she woke up late at night, walked into their room and peed on the floor while laughing, when they asked her in the morning why she had done that, she said she didn't remember having done such a thing
I agree, could be just sleepwalking. Apparently I would sleepwalk a lot as a child. Sometimes I would talk rudely or use my parents closet as a urinal. My parents put me through tourette's testing and psychologists. I grew out of it eventually. still sleepwalk and sleep talk occasionally but no more urinating weird places and usually not rude. Personally I think it was stress/anxiety related.