Wow you seriously think that someone selling their kidney to feed their kids is done with their own free will?
As a society, we should put measures in place to make sure no-one ever has to make this decision (easily accessible food banks, state support etc).
And we should make sure anyone making that decision is aware of the alternatives (and implications) and not suffering from a medical condition which is impairing their ability to make decisions.
But the fundamental choice should be down to the adult in question.
But the way to do this is by adding new, better options, not by restricting existing, 'bad' options.
Lets allow people to get exploited because it might not be a problem in the distant future? Or, work from both ends. By restricting bad options at the same time as we add, best we can, the good options.
We shouldn't allow people to get into a situation where they have to choose between losing a kidney and their children starving.
But - if someone is in that situation - it makes absolutely no sense not to let them decide which option they find least worst.
By doing that, you're only making things worse.
But such transaction happens within a lifelong game with at least two players. Changing the rule of the game can influence their game tree and their behavior, far before they get into a situation where the change applies.
For the kidney trade, banning it might discourage potential organ-buyers from 'check-mating' people into selling their kidneys. It also might nudge parents to work harder since they have one less things to sell off.
I acutually have no opinion over banning organ trade. I'm just saying that it could make some sense.