First: do not, under any circumstances, try to "trigger" them. That's a great way to end up in a reddit thread a decade or so down the line complaining about the child's mentally abusive parents - and they wouldn't be far off. Trying to "shock" them into good behavior would do nothing except make them fear you, which hopefully isn't your goal, and encourage them to get better at hiding their misbehavior. This never turns out well. "Getting their attention" is something you shouldn't be doing by hurting them: it's something you should be doing by talking to them, as human beings, and teaching them, by example and by words.
give them a smack
That sounds like it's also part of the same problem. Physical punishment does not solve any problem, other than, again, making them fear you. Is this what you're hoping to do? Physical abuse as a disciplinary method is never acceptable, not only because of the real harm it does but also because it doesn't work for the most part. Your goal should be producing a child who wants to behave properly, not one who acts well-behaved due to fear of physical consequences. Again - this is another human being; treat them that way.
It got to a point where we told the boys we had sold their DS's, which they believed for several months`
So - you lied to them, and they caught you on your lie. At this point they haven't even done anything wrong - at the point you told them they sold the devices, they no longer had the obligation not to play with them, after all. At least, that's what I'd have argued as a child. Lying to children is never the answer for things like this - I'm generally opposed to even the minor lies (Santa Claus, doggie went to a new home, etc.), but here it's far worse: in a disciplinary setting, you lied to them. That tells them that you think lying is okay.
Let's start over here. What you have is two children, about the same age as mine I might add, who go behind your back to get away with things they shouldn't. There are two things you need to consider:
- Immediate approach (what do we do when we find them doing it)
- Long-term approach (how do we solve this over time)
The immediate approach should generally be to stop them from doing what they're immediately doing, and put some appropriate barriers in place to help them stay on the straight and narrow. This dovetails with the long-term approach, which is to teach them why they need to follow these rules.
I recently had a similar issue with my seven year old, where he figured out our parental control passcode that we used to enforce his "downtime" at night (he'd read all night if he could!). How we approached it:
- Immediate: Took his device downstairs; changed our passcode; reminded him he wasn't supposed to be reading, and why [lack of sleep = sad child tomorrow].
- Long term: Talked with him about why it's important to sleep. Talked with him each day for the next several weeks (up to present) at bedtime, reminding him that his reading time will end, and asking if he's going to have a good bedtime.
Notice we didn't have any "severe" consequences. Our consequences were all directly related to the behavior: stopping the behavior, making changes to help enforce the behavior long-term. That's really what is needed: to reinforce the good behavior, and put rails on the edge of the road to make it a little harder to go off. We didn't have any illusions that our passcode was a perfect solution; nor were we particularly surprised that our child eventually figured it out. For the most part, that stuff is there just as an alarm that tells them it's time to go to bed, and avoids some arguments; as they get older we'll remove these, and let them control their bedtime more directly.
Also note that our child did not lie to us when we confronted him with it. That's in part because of how we addressed it. It wasn't a big deal, and there was no anger or yelling - just a brief conversation about it. Most of the conversation was the next day, when we had the first of the long-term conversations.
We also addressed the bigger picture in those. I pointed out what bad consequences are possible of someone who steals an administrative password later in life. I reminded him that he has other routes to take if he doesn't agree with a limit. I explained how stealing our passcode made us distrust him, and how that lack of trust leads to more limits - while his brother, who has more freedom, has earned that through keeping our trust.
In your case, I think you need to re-establish trust on both sides. Talk to them, kindly, about how they are hurt by this behavior. Show them the negative consequences they create for themselves - how tired they are the next day. Try to work with them to get to where they understand why you have rules here.
Then, set up a reasonable schedule, that includes them getting to play with their devices. Totally removing the devices is a mistake: it doesn't give them a way to recover. Set up a schedule, say, 1 hour a day, whatever is reasonable for your household, and then set up rules around that - if they use their device over their limit one day, they lose some (but not all, generally) of the time the next day. Losing some, but not all, is a far better lesson in my experience: losing a whole day, they'll just hate you for it but mostly forget it long term - but losing half an hour from their hour? Much more noticeable when it happens, because they have it for some time and have the extra reminder of having to stop. They also have an extra positive reinforcement - when they do successfully follow those (shorter) limits, they have the positive reinforcement of the next day getting the full time.
If you don't feel comfortable just giving them their normal time after all of what's happen, compromise here: give them a way to earn it back. Tell them they can have their normal time each day if they do an extra chore. Have this be time bound - say, they have to do the extra chore for the next few weeks. Or, have the limit be half of the normal time for a few weeks.
But honestly, I'd suggest not doing that, and just having everything be a fresh start. Lying to them about the device was a big mistake on your part, and you can use that as a good example for them: you own up to your mistake, and tell them let's start clean.
You could also set up a reward chart, going at this from the opposite angle. On days they follow the rules, they get a star, and a number of stars later, they get a full day of playing, or something equivalent and appropriate for your family.
The idea, regardless of approach, is to give them a mild incentive to comply - but mostly to teach them why they should comply, rather than just having it be a "because I said so". Children who believe the rules make sense, will follow them; children who think the rules don't make sense, won't.