Ultimately, then - this is about how the children are treated in their slavery - not if they're being treated as slaves. (Because, as we've established: all apprentices are slaves.) If the "Gifted Child Agent" is going even a marginal job, then he's better than house Tytalus - and the crusading wizard has an entire House to vent his righteousness on.
From the more pragmatic "this is my job", I can actually see that as being set up as a legal defense, to prevent someone from taking the children. "I'm not claiming that you can't take him as an apprentice. I'm claiming that, as his legal guardian/owner, you did not negotiate with me for his service, as you would with any other parent. I am more than happy to make that accommodation. My fees are clearly established in the Tribunal. Here's the bill."
But, as others have pointed out - that's a more mercenary variation of what Thebes does, and would require some politicking to get it to stick. It's running up against the ruling of "a magus may take an apprentice - and once they take it, it becomes their property." idea. The only rulings against that would be in the "interfering with the mundanes" or "bringing ruin to my sodales" - such as by kidnapping a noble's child, or something. And to use THAT as a legal defense, the Agent would have to threaten to go to the local lord when the other magi took the child.
Hm...potentially, that may work if the Agent somehow had an agreement with whoever he got the child from that he would re-sell, and give the original owner a portion of the profits. In THAT case, if the child was taken as an apprentice, the Agent could claim to be legally responsible to report the crime (and thus bring mundanes into the scenario)...unless that reasonable fee was paid.
In that scenario, it may be more justified for the Agent to be a mundane himself - similar to how magi get around the "don't sell magic items to mundane" clause. Or in this case, it's the very fact that he's mundane means there is an additional layer of legal protections involved.
EDIT - just read your last post - it's an interesting legal distinction of the child NOT being an apprentice, and therefore "just" property to be compensated for in a civil suit...which is a similar line of argument that I was using, although I was using the "don't interefere with the mundane" clause to force payment.