There are much worse real life scam traps than the couple dozen or hundreds dollars worth of mtx (if your kid has that much allowance on their device.)
If he only have 20 a month he would have to come up with strategy to get what he wanted and then if he wants extra he has to work for it.
Your analogy would be like give your kid access to your bank accounts and CC without check. Learning opportunity cost without limited resource is a poor example.
You do realize that there are much worse scams irl for adults than gacha games right?
Unfortunately, human do learn "better" through experience (subject to personal trait that falls under a statistic distribution), and because of that a
is much much better than
I could afford to let my kid learn opportunity cost using the mtx, or dollar store toys, actual gacha eggs, etc once I start to let them handle their own allowance. They could also learn a bunch of marketing strategy that applies to them to make them dump allowance when talking with me later requesting to get more money to buy whatever they currently wanted. I'd rather spend that money as learning cost, than whatever forbidden method and then when they started working they dump their entire paycheck on those or even more expensive stuff because the sales convinced that they "can".
So yes, fuck them kids, let them stumble when they are short, let them trip to learn to pay attention on the road, let them whine for a month why they can't get extra to buy skin or whatever latest and greatest trending shit that some of their friends have because their physical or virtual piggy bank is empty. Fuck them with all the tricks at much smaller scale and let them learn from it.