"Naughty"? As in kids committing crimes with crypto online? The odds for this are very slim. But if you mean, making unauthorized purchases (purchases not allowed by their parents), that's another story. With how transparent popular cryptos are (Bitcoin, Ethereum, Litecoin), it would be easy enough to track kids' financial activities online. In the case of privacy coins, it's almost impossible to do such a thing.
I seriously doubt a kid is going to know how to use a privacy coin, though. Especially when many privacy coins have a slight learning curve. Monero is hard to understand by those coming from traditional cryptos. Grin is even much harder to learn. Besides, merchant/business adoption for privacy coins is limited. Bitcoin would seem to be a much better option for kids, imo. Adoption is just getting started, anyways. Kids might either use CBDCs or crypto massively in the future. Only time will tell.
If they are committing crime then yeah it would be easy to track and cops will find it, but if we are talking about hiding stuff from their parents? Then privacy doesn't mean anything, you will not know what your child is doing at all.
I know this because I was once a kid with a computer and I can tell you that I have done so much that nobody knew, nothing illegal though, I never did anything illegal, just things that were banned by my parents. A simple like "play games more hours than allowed" is simply known all around the world.
This is still a bit hard, because we are talking about hiding from parents and it is not always that easy. Depends on the parents and their age of course, some parents do not know tech that much and you can hide from them easily which will not be an issue. But some parents are quite smart, and if you are a 15 year old today (since topic says kids) that means your parents could be as young as in their 30's, because if they had you at 25, they are 40, but in most cases they are probably little over 40.
Those people can understand tech very well and research what you are doing with your PC when you are not at home, and they can easily find everything they are looking for. This is why, as long as it's legal, I think it shouldn't be a problem.