If I see it happen, I can absolutely tell the difference between my kid breaking something out of carelessness vs. intentionally. If I don't see it happen, it's a reasonable assumption that it was simple carelessness unless kid was really angry at the time, in which case it's quite possible kid was taking out rage on the object. (We still have a story in my family of when my brother was really angry, went upstairs, loud thump, and brother came back down pale and shaking and said "Daddy, I broke the wall." He'd kicked the wall and learned that drywall is fragile; Dad had him do the repair under Dad's supervision.)

So if LW weren't already biased against the step-grands, I'd be inclined to take them at their word that the boy is breaking things intentionally. Given their horror at eating pizza with one's hands, though, I'm not seeing LW as a reliable narrator.