To sign whistle, hold your index finger and thumb to your mouth and pretend to blow on your imaginary whistle. This sign can be used both for the actual whistle contraption and the act of whistling by only using your fingers held to your mouth.
Teach your toddler the whistle when they ask you about it, when they hear it on the school fields or gym for the first time, being used by the coach or referee.
Teach this sign along with the other flash cards pertaining to sports, practice, and playing signals. While the flash cards are useful for reviewing the signs, the best situations to teach the signs are always real-life experiences.