This is usually accompanied by a silent inner pleading, "Please don't do it again; don't make me a liar. I don't want to leave this slice of paradise any more than you do.'
Better to send them down the beach, to throw sand at no one but themselves... but that's assuming there's no wind, and no one else on the beach. Yeah, right.
Or we could bribe them, 'If you splash your sister one more time, there'll be no ice cream.' (Just as likely to end up a lie.)