On April Fools we were treated to a whole range of little tricks -- cellophane over the toilet, little boy undies in my lunch bag (clean of course), fake children in the bed at bedtime (real children running about). But there was one trick we missed, one that took us totally unawares the next day.
BUZZZZZ (re-enacted brilliantly by the Daddy)

hiding under the cushion: battery, wire, and one loud little buzzer! Arranged so that any pressure on the cushion would complete the wire circuit and make the buzzer go.
Clever little boys.