Not that this implies the quiet ones are more likely to blow than the troublesome ones. Rather, since warning signs would often cause kids not to be described as 'quiet ones', the statement would seem to boil down to "Kids who don't give warning signs are more likely to blow without warning than those that do give warning signs, since the latter by definition can't blow without warning."
One thing parents can do when their children seem unreachable is PRAY for them.
Reminds me of my own personal favorite: "only strangers talk to strangers".