Donald F Averill 
The Niffits [EPUB ebook] 

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.