"The Fluffy Pink Toadstool"
is a short story by Diana Wynne Jones
"Mother was always having crazes. Since she was a strong-minded lady, this meant that the rest of the family had the crazes too - until, that is, Father put his foot down."
Mother decided to go through a hand-made phaze which involved her family making a lot of sacrifices. This leads on to a natural food craze and she insisted that the family pick their own. Whilst out in the woods Tim, the youngest, is given a bright pink toadstool by "a funny man... with trousers like Father's." The consequences result in Father putting his foot down.
It is in several short story collections: Warlock At The Wheel: And Other Stories , Everard's Ride, Unexpected Magic and [The Cat Flap And The Apple Pie]?.