Yes, American citizens will generate tax revenue. But they are going to use these resources now, not when their children are of working age. By this logic, we should let a billion people in every year because their children will generate tax revenue one day.
They will spend money and pay sales tax but I have a hard time believing that they will actually pay any income tax. Using H&R Block's calculator, a person making $25,000 a year, with 2 children, and had $2000 withheld, will receive $9,310 back if married, and $7,863 if they are head of household.
As soon as they have children, they will be eligible for welfare. Their children will be in public schools. Remember all of the teachers who complain that they are underpaid, work too many hours, and have to spend their own money for supplies for their students? Will they get paid more when students are added to their school? When new teachers have to be hired, is there going to be more money available to the school? What percentage of the new immigrants are going to buy(adding more money to schools through property tax) as opposed to rent(property tax has always been paid by the owner)?