The Santa Anna Gold is a short story, less than 6k words, and it was first released in the Time Travel anthology Synchronic: 13 Tales of Time Travel which is available here as well. It is a short story, so please do not buy it if you are looking for a full-length book.
A father tries to help his son—the only family member he has left—pursue his interest in finding the legendary Santa Anna Gold. The only problem is: the boy wants to go back in time in order to locate it. Sometimes the things a father will do for an only child are limitless.