The Niffits — Donald F. Averill

Curiosity has caused twelve-year-old Roxy Maxwell to investigate a strange occurrence that took place a couple of days before she is to begin the seventh grade. Her father and her best friend, Dexie Chappell, are enlisted to help her solve the coin mystery. Dexie, a bookworm, contributes ideas and a karaoke machine to assist with the detective work. Dean Walker, a fifth grader and neighborhood spy, has to be trusted with the Maxwells' secret. The investigation team must maintain secrecy or the news media and scientific community will descend on the Chappells' property, ruining the plan to return the aliens to their home country.