I've told my children from the time they were toddlers/preschoolers, "We don't keep secrets. If anyone ever says 'Don't tell your mom; it's a secret,' the first thing you do is tell Mom, because we don't keep secrets."
We do, however, keep surprises: a birthday surprise, a Christmas surprise, a surprise party.
Kids know what a surprise is. Giving them two different words helps them make the mental distinction on when they should tell, and when they should not.
What safety lessons are you teaching your child today?