Being paid changes the dynamic in a deep way from being a hobby. It opens up the floodgates of manipulation and abuse.
How is it at all tertiary? It's the primary point: we don't allow children to work for pay because of the potential for abuse; children get paid to act; many child actors are abused as a result of extracting money from them and their labor.
We make an exception because it is convenient, not well understood by the public, and we all benefit from the resultant entertainment.