I think children are entitled to have their childhood for as long as possible. The idea of trading to make a profit is something only adults with responsibility think about - let them be children while possible.
In my view, it depends on the interest of the child and their age, as well as their reason for learning such skill. For instance, rather than having them work as a morning-paper delivery you might want to say that there's a different way to make money etc.
It is important for them to understand the value of money as well as the process of making it, and only exposure and experience will equip them for what's to come in their adulthood.
In a nutshell, it all depends on the parental-child relationship, interest and age of the child.
You are thinking about little kids, "your child" doesn't mean just 3 year old kids, it also means 10-15 year olds as well, hell even 18 years old people are "usually" not great with money as well. I get that there are few who are better than all of us, but usually people are not great with money until they are well into their adulthood.
I have been in crypto since I was 23 years old and now I am nearly 32 and I haven't made millions from it, which I could have if I was any smart but I wasn't and I didn't yet for the past 2-3 years I have made more money from crypto than all the money I made in my entire life.
So, I can easily say that it is really not that easy for someone to be smart about money without someone teaching. So you can start when they are 10 or 12 or 15 or even 18 if you want to, doesn't matter, they are still your children, even if they are 50 year old they are your children but this mainly focuses on the period where they are still living with you and under your guardianship.
