But on the other hand... what would those "children" do, if they were not allowed to work? I guess there are no schools around having empty seats.
Not so easy. And they would not work, if their families would not need the money. So if you just take this money from them, will their lifes become easier or better?
And how big do you think are the chances that the respective country will spend money onto those families to compensate the loss of the child income? In a scenario where exactly this state suddenly earns less taxes, as there are now less workers working?
And how would realize their worth do them any good? If they woul start to strike for better salaries, the company owner would just move to the next country and our striking workers will have 0 income from this point on. How would this make their lifes better?
And the workers in the country the company owner moved to will now very likely not even think of a strike. They can see in their news what happened to the guys who striked. This was not nice and the new guys rather work hard 14 hours shifts in the factory, as this is still better than their lifes before.

Don't get me wrong: I don't think this is good or should be this way. But it is much, much more complicated than one usually thinks.