score:21
As with today, there were many different terms used in medieval times and what a little girl would have said would have depended very much on her upbringing / social background and perhaps the immediate environment in which she found herself.
She could just use the verb piss as it was not considered vulgar until circa. 1760. Here are a few possibilities (nouns) where she could go (this is one verb used with at least some of the nouns below):
Medieval Chamber Pot (1300-1500), Yorkshire Museum collection
a gangpytt (‘going-pit’), gangeen (‘going place’), gangsetl (‘going-seat’), and gangtun (‘going-yard’), or an utgang (‘outgoing’), forthgang, and earsgang (‘arse-going’).
Source: David Crystal, 'Words in Time and Place: Exploring Language Through the Historical Thesaurus of the Oxford English Dictionary' (2014)
Unless Shakespeare (writing in the early modern period) used an anachronism, water was used in the middle ages to refer to urine; this appears in his play Henry IV (d. 1413). Jonathan Swift (1667 - 1745) is cited by Samuel Johnson, "Go to bed, after you have made water", but I can't confirm if this was used in medieval times.