Sure, but if you found something right away, there's a different sense of loss compared to losing it and not finding it for a day. I'm not expecting the manipulating to be fondly remembered, but the point can be emphasized by keeping the object hidden

It can also be emphasized by saying, "here's the object you thought you lost, but now you can't have it back because you were irresponsible."

This seems like an unreasonable expectation, to talk to kids directly

You didn't understand my point. I'm not asking you to have a moral debate with kids. I'm saying, don't lie by omission and manipulate them. I knew daaaaaaamn well at 4 years old what it meant to not get a toy back because I had done something bad.

but really, kids will make mistakes even if they do understand things

Absolutely! This is why there's no advantage to being one of the crappy parents and thinking there's something to learn from manipulating.

In my context, the "liking" being tricked would at best just be potential appreciation down the road, like how when people get older they might look back and say "oh, I see why he did that now, I'm glad he did".

This is too relative to have any real meaning. Objectively, I had a far worse upbringing than many people. Emotionally, I'm about as stable and happy as any of them. If a person's point of reference is being manipulated, they are less likely to object to it as an invalid tactic for dealing with people because they don't know how much better it can get.

Obviously adults don't like being told "I told you so", but you also wouldn't try to discipline another adult. There are many things I believe a parent should do that a child won't like

Did you consider what you're arguing for before you wrote this? What you do has to have a purpose beyond your own personal gratification. I told you so's don't change the past or future - it's expressly to rub someone else's nose in the dirt. I outgrew "I told you so" before I left elementary school. This is quite possibly the most juvenile part of your post and the bit that leaves me the most disappointed because you seem to think this kind of self-gratifying behaviour has a place in a kid's life.

I fully expect my kids to have access to expensive things, like a TV or tablet or computer, and I also think being able to trust them is important too

Unless you're letting your 7 year old go on field trips with the tablet, they aren't going to lose these things. We're talking about items that can be lost. Yes, if you are giving a young kid something small that can be easily transported and lost, it is YOUR fault for making a poor judgment call. To think the solution is to divert responsibility to a child is parental ego gone wrong. Accept responsibility for it.

As for the retainer, you'd be a mighty asshole for withholding something like that, so I'm sure you wouldn't. If it's a necessity, then you can't withhold it so there's no lesson to be learned there. If you want to teach them responsibility, escalate slowly. Of course, kids may value things differently than the actual dollar value of the item, but we go back to my earlier point of simply being direct with them. You can withhold said trinket but they aren't being lied to.

Regardless of age, if you approach someone with a loud voice and hurtful words - even if they did do something to provoke it - you KNOW they aren't going to be receptive to it. Whatever valid message you think you had is going to be dampened or lost completely in these stupid mind games you're trying to justify. So much more can be accomplished - yes, even with a kid - when there's an uninterrupted pathway between A and B.

I'm sensing a theme here, and I'm guessing you're basing most of your opinions here on your experiences

Okay, Dr. Phil, you admitted throughout your post that parents like fucking with their kids. You also admitted previously that you're inclined to being an "I told you so" kind of person. What I said is putting your thoughts together so that you can understand how unflattering the reflection really is. As for what happened in my family - I actually never clarified or gave enough detail for you to draw a conclusion that would put you at ease of being dismissive (note: I don't actually care what your intentions are - your statement was condescending). But, FYI, the specific things we're talking about here were infrequent enough.

Also, keep in mind that your desire for good results has no bearing on the actual morality or results of your actions.

I'm predicating all my opinions on having a good relationship with my kid

If you asked my parents, they'd probably tell you they have a good relationship with me. I feel like you're one of those adults who's already forgotten what it's like to be younger, and as such are unfortunately oblivious to how your actions as an adult can impact someone on the receiving end. Life experience only has to do with some of our actions and teaching moments as adults. But, as weird as it sounds, a lot of parenting is just remembering how to treat another person, even a wee one. We get callous in our 'old' age - not necessarily abrasive, just toughened. But since we're less impressionable, each bad or hurtful act brought against us leaves a much smaller footprint. "Youth can not know how age thinks and feels, but old men are guilty if they forget what it is to be young."