Well yeah, I do all those things too, and I'm sure most players do, but isn't contradictory with the stance that a mankad is no different from a stumping or from running a batsman out who hasn't grounded his bat behind the line in normal play. Its just another perfectly fair and valid way to catch out a dozy batsman and gain a wicket. Cricket is a game of concentration, if you lose your concentration, you lose your wicket.
Would I mankad a 13 year old in a friendly game? No, not even with a warning. I wouldn't stump one either, and I probably wouldn't appeal for an lbw.
Would I encourage one 13 year old to mankad another 13 year old in an U13s league fixture with no warning required? Definitely - its a league game, they need to learn what is and isn't acceptable, and if we want to get rid of this daft old-fashioned idea that a mankad is somehow unsporting (which we do!), now is a good time to teach that lesson.
Fair enough, I completely appreciate that different opinions and interpretations exist. Allow me to propose a compromise:
Imagine you and I are captains or coaches. When we walk out to toss up, you say to me, 'Justin, please understand that should your batsman back up too far, too early, we will Mankad him without a warning on the pitch. I will also be advising the umpires before play begins.'
This, for me would be both fair, and entirely within the Spirit of the Game. I would let my team know, and there would be no hard feelings if the event occurred.