I never saw the full film, just a uk trailer, dated maybe about 1990. I thought at the time "I'm gonna watch that" but then the years intervened.
I've had a search through this site but no joy.
What I recall is this kid aged approx eleven is gaping in horror because he learns his parents (step-parents maybe) are shape changing aliens/ demons
They proceed to flip through a few changes while laughing at his reaction, one that stayed in my memory was "you called us butt-heads, well look at this!" and both then change their heads into people with big butts from the neck up.