I think that wall is just a warning for the ilegal inmigrant inside US, its just a expression to caused fear and sop the huge amount of ilegal mexicans into the border.
The flow of illegal Mexicans must be stopped. They are not paying the taxes, and they are exploiting the welfare system by claiming child benefits for their dozens of children. Cheap labor is good, as long as they are legal.
It is a problem of the government. Children's allowance is not paid to the illegals is not an argument. Furthermore, let's understand why they are working illegally. In my country the third part of the salary should pay in tax. Who wants to honestly pay? You need not to deal with the phenomenon, and to make sure that everything was favorable.
Even if the federal government want to put an end to this practice, the deep-blue states such as New York and California, where most of these illegals are residing, will oppose. California is perhaps the most pro-illegal immigrant state.