You're putting way too much emphasis on a kid's choice to play pokemon based on or not the value of trades they've gotten. Most of the kids you see getting into Pokemon aren't going to be there in their teen/adult years. Look at the number of people selling cards from Base Set-Team Rocket on ebay for the last decade because that's where they stopped as a kid. You know what will affect whether they stay with us or not?
1) Their parent's willingness to support the hobby. I'd say this is the #1 reason kids stop playing, the parents won't buy more cards or don't have the money to take them to tournaments
2) Kids grow up and find different interests. Does anyone remember what middle school was like? *shudder* In our league we lose good players in the 10-12 age range who quit because Pokemon isn't cool anymore and their friends are making fun of them.
Out of all the people I played Pokemon with when I was 10, I'm the only one left. I've made plenty of new friends since then, but out of all the people I played with back in the day, I'm all that's left. A lot of my friends didn't have enough money and their parents couldn't take them to big tournaments all the time, so they eventually quit. I was lucky that my dad took me to leagues/tournaments and bought me packs for birthdays and holidays. I can't name a single person who quit because they suddenly had an epiphany on the bad trades they had done because if it happens, it's incredibly rare.
And honestly, making assumptions about a person because of what they do or don't do when trading pokemon cards is rather immature, too. You're telling me you've never taken advantage of an opportunity, ever? Everyone who gets a steal on something on ebay should pay what the real market value for that item is? You can't compare theft to getting a good deal on a trade.. theft results in one person being sad, and one being happy. Kids aren't going to trade with you if the trade would make them unhappy. One man's trash is another's treasure. Under these assumptions, any card vendors (including some reputable ones on this very website) are doing unethical things because they're trading for profit.
I think we need to stop putting kids into such a protective bubble all the time. They don't learn that way; no one does. It's like when you're taking a math test and a question calls for the amount of dollars required and you forget to put the $ sign in your answer, even though it's right. When my professor/teacher marked that as wrong, I learned to pay attention and never made those mistakes again. When I was a kid and dropped my oldschool game boy on the floor and the screen cracked, and my dad wouldn't get me a new one, I learned to take better care of my things and I never had any incidents with any future game boys. When the kid gets ripped off and it affects their ability to get something they want, they'll learn to be more careful, and if their parents allow it/show them, will use good internet sources for price values. If you just casually tell a kid about values, they might use it for that one trade but they'll forget the next time. No lesson learned. And how are you ever going to expect a kid to look up the card values online on their own if they're always being protected by someone when trading? Why bother if the work's being done for them?
That's just my opinion on the subject anyway. I won't actively seek out kids/juniors when I'm at an event because I rather trade/talk to people my age, but if a kid comes up to me and offers me something ridiculous, I won't hesitate to accept it. If they're offering, they'll clearly be happy with it, and I think the odds are in my favor that the kid won't be playing the game in the future anyway; not because of me, but because of external reasons I cannot control. Their cards will be sitting in a shoebox in their attic like so many thousands of (now) adults who used to play.