Perhaps "low slung", this tends to be a style choice by teens, to wear jeans with the waistband low perhaps showing the waistband of their underwear (but not so low that their butts show)
For a little child you would just say "Jonny, your shorts are falling down" or some similar expression. There are no specific words, you would make up your own sentence. In many cases you would leave part of the sentence unspoken. "Jonny! We can see your butt!" (implying from the context that the reason is that the rubber is weak etc.) When talking to little children, families often develop their own expressions: "Jonny, you're wearing your falling-downers again."
For an adult you would not normally mention anything. However there is the "workman's bum" (butt-crack exposed by a fat man bending over)
So there is no "common term" but you could use a sentence like the one you suggested.