What can I do to prevent two well-behaving siblings from turning into a maelstrom of destruction when combined?

Question

“Yeah!” said James enthusiastically. “I don’t mind sharing a room with Al—Teddy could have my room!”
“No,” said Harry firmly, “you and Al will share a room only when I want the house demolished.”
("Harry Potter and Deathly Hallows", Epilogue)

My 2 kids by themselves are reasonably well behaved and mature for their ages. They can be left alone in a room/floor (while the parent is working or doing chores elsewhere in the house) for hours at a time, with rarely a misbehavior happening. Especially the older one.

However, like Oxygen and Hydrogen, when combined, they become a maelstrom of destruction - it's almost assured that by the end of 1 or 2 hours they would at best turn the entire house into complete mess, and at worst do literally several hundred dollars worth of damage (usually, in a way that is so quiet that the only way you KNOW something is wrong is by absence of sounds from them).

So far, the ONLY solution that ever worked was to forbid them to play together unless someone adult is in the same room to supervise - which is not always feasible.

This isn't a solution I like, so I am looking for a better alternative of how to manage the issue than the Harry Potter approach.

We tried:

• Rewarding them for behaving well when together. That only lasts 1-3 days
• Discussions over what should and shouldn't be done (which is 100% useless - they virtually NEVER do such bad stuff when alone, and know perfectly well that it's bad).
• Discussions with the older one over "You're an older sibling, can we trust you to keep an eye out for the younger one and tell her not to do bad things"? He likes the idea of that trust and responsibility as a concept... but forgets about it pretty soon.
• Discussions with the older one over "Never do what your sibling suggests if you know it's wrong" (or when anyone suggests in the first place). Again, perfect agreement... up until it's 2 of them for a significant period of time.
• Harsh punishment for causing trouble as a pair (including being separated from each other for several days).

P.S. No, they do NOT share a room. We do not want the house demolished, thankyouverymuch.

Answer 1

Kids get excited when they are together, and may forget rules, especially if it turns to be a special event (it seems that you don't allow them to play together that often). I have a big family and most of my cousins are individually well-behaved but, there is nothing much to do, during family gathering, it always finishes with a horde of kids running everywhere.

Why don't you play quietly with them a couple of times (for example, board games). And when you have something else to do, you can propose them this activity. They will know how to play and have seen a model (you) on how to behave. They don't need to have this kind of quiet activities every time they play together, and may choose their own games, but at least, when things get messy, you will have an alternative to separation and harsh punishment.

Answer 2

The only thing that ever worked for me is: fixing the problem they have caused. (that is, if beforehand you have told them they would have to fix problems... :p)

If they threw away their legos all over the house, then they take a bucket each and get them all back. (that becomes interesting when they have mixed all their puzzles together, I am telling you! but then it also fixes my other problem = "mum I am bored, there is nothing to do in that house" -> well, spend a few hours doing all your puzzles, that will keep you busy)

The other day, my 5yo walked all over the house with his muddy socks, there were footprints everywhere. I handed him a pack of baby wipes and wish him good luck. I hope he will remember to remove his dirty socks next time he goes playing in the mud. It may takes another pack of baby wipes, though.

Also, they could try and fix broken stuff (they won't do it, but at least they will have seen how hard it is to fix things). If there is financial consequences, tell them how you are going to get the money back (by saving on lollipops till the end of times or whatever). I wouldn't let them try the rules near my new computer for instance, but they would still have to be in a challenging environment anyway, so find something.

And well, you basically ask: how can I get them responsible together? Then I'd say you have to let them try. They will never learn how to play nicely together if they are not allowed to. I was giving these advices above (fix problems), based on what we live in our house, but these are the results of individual behavior. My three kids (6yo, 5yo and 2yo) sleeps in the same room and have also another room to play in. They play nicely together, because they have to, that's in their own interest. (the same goes for sleeping at night, why would they bother each other?)