As I said back when I was discussing the odd way in which April's control of her belongings wasn't really up to her, the general idea seems to be that John and Elly had arrogated to themselves the sole right to decide who is allowed to own and dispose of any possession they might have. The hand that angrily claws at Mike's toys because the reminder that her house has been invaded by a child is the same hand that greedily clutches onto toys April would allow others to enjoy because her hand is the only hand that matters. Elly and John might not see themselves as monsters but they have no real moral standing to do what they do and it's rather damaging.
This, I should say, is why it is that left to their own devices, the children cheerfully wreck things that other people would prefer not to be molested with because they don't and can't understand that merely because they're bored, they can't braid ties or waste office supplies because other people actually happen to be real and have feelings that matter. Not that it's their fault completely; for other people's feelings to matter, they'd have to have parents who think that their feelings matter.