Panel 1: Mike makes the foolish mistake of forgetting who his parents and assumed that they were going to let him have the run of the place while they all went to Vancouver.
Panel 2: When he says that they don't trust him, Elly says that they do while John says that they don't trust the situation. In an old-time British game show, the announcer would have proceeded that with "The next object is a lie. A lie."
Panel 3: This is because their talky-talk about a fifteen year old kid alone at home for two weeks with a full liquor cabinet and cars says that the situation they don't trust is them having a teenage son to worry about.
Panel 4: If his Gordo-like blank stare as he expresses confusion as to what he might get up to left to his own devices means anything, it means that they should be worried about his standing around like an idiot looking stupid as the place falls down around him instead of his getting likkered up and trashing it on purpose.
Summary: John and Elly fear mayhem and destruction because they fear their children. What they should fear is his standing around looking stupid and letting the place fall down around his jug ears. Mike has no initiative and only does things in reaction to outside influences. Hmmm. Should have taken him to Vancouver too. Get him away from THAT GIRL.