In my country (Germany) it is not legal to do business with children in the age 0 to 18.
From 0 to 7 it is forbidden, and from 7 to 18 there are some minor differences, But overall and away from everyday life things, in Germany it is not allowed to sell children (age 11, 12, 13) and young people (age 14, 15) bigger things. For example, if I want to sell a television set to a 13 years old person, it is not allowed and I have to give back the money the 13 years old was paying me for the TV set.
In the case a 13 year old is selling me something with Bitcoin, it is easy to handle. I get the TV set back (from my example) back, and I have to pay back the Bitcoin or the money therefor (at the market price).
But my question or my reason to discuss:
If a 13 year old children is mining BTC this is not allowed in Germany. The question of the hour is, not just for Bitcoin - for all POW coins: How to protect chuldren from mining coins? Is there a statistic about how many coins (BTC) were mined by children or persons under 18 years? And how to compensate this unlawful behavior?
Is there a mining protection for children on the Bitcoin core?
The same way a kid can go to a river in the German alps to sip through stones and sand for some dust of gold, he/she can try mining on their laptop or gaming rig / whatever. What should be illegal about any of these activities? Does it need specific protection? I don't think so.
Keep in mind, without the right (expensive and hard-to-get) ASIC hardware you will pay 10000x or more for energy consumption vs. the returns of your mining efforts.
With current consumer-prices for energy in Germany I doubt it's even possible to stay net-positive if you own some of the latest ASICs.
Regarding ETH/similar mining with graphic cards, it could be possible to earn a few bucks per month. But probably any newspaper-distribution-job once a week will be more lucrative for any teen unless they could afford 10 or more of the latest graphic cards.
All in all I don't really get what you are concerned about. Legally and child-protective-wise... Nothing to worry about here.