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That's funny!! But why?

Plus....wouldnt that mean little bits of food being spewed over your apples and work area?

XD
Peace

As the Dad, it gives peace of mind, if they are whistling they aren't eating, so you can do whatever you are doing without a worry.
If the whistling stops, they are up to mischief, and it is time to look.
It is also self-policing, as you say, who wants to eat food your brother/sister has splattered with half-chewed food. they will tell each other off for doing it.
If you do the briefing with a stern enough voice, you can sit in another room and listen, trying not to laugh out too loud.
there has to be some fun in parenthood you know,