This should be limited to giving advice (education, warning, explicit consent), unless there's harm to third parties.
Because, you know, certain actions and even thoughts can lead to eternal damnation in Hell, according to what a society may think. Would you prefer the society to hold you off from that?
A child is not a fully autonomous person. I would of course take the loaded gun from the child, unload it, and explain its dangers to the child.
Money, in any form, may be as dangerous as a loaded gun, trading stocks or not. Most adults are careful with money, as they are with loaded guns. The problem is that some parties may try to make trading stocks (even leveraged) look much easier and safer than it is. It's like giving somebody a real loaded gun, while making it look like a toy gun, safe even for a child. And this of course needs to be regulated: not the trading, but the disclosure. This is not a toy.
Who are you to decide that a child is not a fully autonomous person? Sounds like you're imposing a normative rule based on societally derived presuppositions of right and wrong, which is exactly the point. We're just haggling over where the line should be drawn and you think it should be drawn somewhere further back than others do, but there's no truth to be found here.