Since I mentioned it, I am incumbent to demonstrate the proofs: Is Whistling Haram? Well, google is showing me that lots of folks believe it is haram for varing reasons (“My auntie told me not to whistle in the bathroom. . . . “I told my brother whistling is haram and he wants the proof”), but I’m not finding much by way of daleel. So I hate to ask, and know it is in bad form, but if anyone has anything more specific that maybe isn’t available online-please share. Otherwise this is one view that it is disliked, though I have seen others say it is haram for this reason and it isn’t the reason that I was thinking made it haram):
Shaykh al-Islam said in al-Fataawa (11/569):
“I know that in the ‘golden age’, the first and best three centuries, in the Hijaaz, in Syria, in the Yemen, in Egypt, in the Maghreb, in Iraq, in Khorasan, none of the religious and righteous people, the ascetics and those who worshipped Allaah much, would gather to listen to this whistling and clapping and drum-beating and so on. This was innovated after that at the end of the second century, and whenever the imaams saw it, they denounced it.”
both clapping and whistling are disliked
“Their Salaah (prayer) at the House (of Allaah, i.e. the Ka‘bah at Makkah) was nothing but whistling and clapping of hands”
The Standing Committee was asked about men clapping when they play with children, or children clapping to encourage their classmates. They replied:
This clapping is not appropriate, and at the very least it is intensely makrooh because this is one of the characteristics of the jaahiliyyah, and because it is something that is done only by women, to draw attention to a mistake in the prayer. And Allaah is the Source of strength.
From Fataawa Islamiyyah, vol. 4, p. 332-333
Children can be encouraged by saying ‘Allaahu akbar’ if they do something that the watcher or listener likes, or one may use other suitable phrases, or raise one’s hands, or raise one’s voice in words of praise such as “Well done!” or “Excellent!” and so on. And Allaah is the Source of strength.