Donde esta casa de pepe?
No, señor. "Donde esta el baño."
And spelling in el español is highly phonetic. Unlike in English, pronunciation of vowels doesn't change from one word to the next.
A = "ah"
E = "eh"
I = "eee"
O = "oh"
U = "ooh"
Thus, your phrase should be "Donde esta la casa de pipi?" (And, yes, they do use the word "pipi" for peepee.)