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.