Credit for this theory goes to Tycio on the Xavier Riddle Wiki.
WMG / Xavier Riddle and the Secret Museum
Xavier is part-robotWhy else would he have a high-tech hoodie and access to portals through time and space?
Sarah's father is abusiveRemember Sarah from "I Am Jackie Robinson"? Sarah acted rather discriminatory towards Jackie in that episode, and so did her father. Maybe Sarah isn't racist at all, and is only acting like it because her father wants her to be, and if she acts that way, he'll keep his beltnote on more often.
The painting of the bald man at the museum is Brad in the pastOne day, when the kids were time traveling, Brad got trapped in the past, and had to grow up there. Eventually, the museum obtained a painting of him.