Look at the mess in the garden—that little devil of a neighbour's child must have been here again.

devil 的本义是"魔鬼"，在口语中常用来形容"调皮捣蛋的人"或"淘气鬼"，带有亲昵或无奈的意味。这里用"小魔鬼"来形容邻居家调皮的孩子，是一种夸张的比喻说法。