Is there any established method to notate for a slide whistle? Or is that usually just done with text explanations? And despite the fact that you can play different tones on such a whistle, is it considered a pitched instrument? Because usually you’d not specify any particular notes to be played but just generally “from high to low” or something like that.
Slide whistle is chiefly used for this comic effect most people associate with it from the Looney tunes and Tom & Jerry cartoons, but you can actually play a melody on it. However, this is pretty uncommon and not very easy to do, because you will hear wrong pitches and most of the time out of tune lines, since it is very hard to get the right pitch.
But if you want the comic effect, just write a note in the staff and write a glissando line down or up, based on your desired direction.