If the children are born in the US while the parents are residing there, then they shouldn't be taken out of the US without both parents agreeing to it.
The issue is that people settle down in the US with their families (regardless of where the children are born) spend years there and then if there is a falling out between the parents, one of the parents tries to take the children back to Pakistan without the permission of the other - that is just not acceptable and not fair on the children - like I said, the courts tend to favor consistency and continuity for the children.
If a Pakistani parent wants to go back to Pakistan with the children, either convince the other parent in the US to agree to the move or go to court (likely to fail if family is settled in the US, barring exceptional circumstances) or realize that the move to Pakistan will have to be without the children.