There are a few key issues.
For one, I want to say that the path between our society and one in which child pornography is legal probably doesn't exist. There's just not a way to broach the conversation that's comfortable enough for people, people have a desire to protect their own kids from the very idea of it.
Second, safety is probably not possible to maintain for children in a world with legal child porn. Children are so impressionable and most people so ill informed about maintaining the sexual well being of a child that probably nothing can be produced that doesn't have a great deal of inherent emotional and physical risk. Weather the benefit from that risk will outweigh the good… hmmm…
In child pornography the key component of the product would be the child and their body, so it's reasonable to say that you risk an industry which is predatory towards innocent children, with various studios doing whatever is within their ability to attain "actors" before they leave some age window, and then discarding the actors after that time has passed. The studios would do well, but the childrens money would have to be left with parents, and that incentivizes parents to entrap children in a career which is easy money regardless of talent level or will.
There would be no way to discern consent, manipulation, or force when speaking to a child brainwashed, gas lit, or simply asked to lie. There would also be no way to ensure the well being of a child that leaves a studio.
The legal exchange of money relating to products centered around children– not a child, like a child actor or a basketball star, but opening up a new industry centered on, in general, children, is rough. You start seeing incentive to engage in things like kidnapping, child trafficking, imagine children being legally taken from orphanages by studios only to be returned after leaving a beneficial age group. I don't believe an agency or union can protect against all these problems…
Sorry I'm rambling, I'm only just thinking this all out for the first time. But I think I can compress it down to a few sentences:
The problem with child porn isn't the literal product itself, it's the life of the child post product first, and second, the incentives monetizing children creates.
You say children are made to play basketball and do cheerleading, and you talk about the danger of those industries, and say child porn would be less dangerous. Well I think for one you make a mistake about the effects of porn on someones personal safety– being seen as and used as an object presents an emotional danger that's difficult to mitigate and a literal danger because of stalkers, entitled and abusive fans, abuse from owners, etc.
Second if child porn and children playing basketball present the same amount of danger, I'd say that instead of advocating for child porn I'd be against child basketball and cheerleading stars. All in all, child porn probably can't be an industry which is largely empathetic towards children.