Bill & Ted Face the Music (2020)
FAQAdd to FAQ
This movie isn't released yet, so we don't know; maybe an explanation will be given in the movie.
The actresses who portray the daughters were both born after the release of Bill & Ted's Bogus Journey (1991); so it stands to reason that their characters too were born after the events of that film (especially since actors and actresses often portray characters that are much younger than they themselves are, such as Alex Winter and Keanu Reeves portraying teens in the previous movies), and hence they simply may be younger siblings to the sons.
Furthermore, the Bill & Ted universe allows for time travel and contains a time travel device. So who knows, maybe we'll learn in this movie that some character had travelled back in time and had somehow changed/affected the timeline such that Bill and Ted have daughters now, instead of sons.
Or perhaps the babies at the end of the previous movie were (or are retroactively written to be) girls after all, and they were merely nicknamed "Little Bill" and "Little Ted", so we as viewers had just assumed that they were boys. Edit