1) all, as in:
the children of all citizens of the United States
2) both parents, as in:
the children of both parent citizens of the United States
3) at least one parent, as in:
the children of a US citizen (all that qualify) of the United States
Of these, #1 and #3 are the least restrictive. #2 is the most restrictive. Under the 14th Amendment equal protection clause, strict scrutiny must be applied to restrict the rights of any person. Strict scrutiny means that a challenged statute must be "narrowly tailored" to serve a "compelling" government interest, and must not have a "less restrictive" alternative. The less restrictive alternative is that it applies to children who have one US parent.