"Prince" and "Princess" are Royal titles for Units. Given that there are no children in Erfworld, and consequently no genetic lineage, It is unknown what constitutes the definition of these terms.
The proper form of address for a Prince of Princess is "Your Highness".
Known Princes and Princesses are:
"Prince" and "Princess" could be honorifics for royal units popped as heir to a side ruled by a King or Queen.