The sort of constitutional challenge you envision turns on the (it seems to me quite apparent) claim that the state system for custody allocations is so constitutionally repugnant in various ways that it simply has to come under federal district court review. If the federal district courts wold follow the logic in federal holdings in every area of law but Family Law, this would be a strong claim.
It can't be overstated that this is a categorically distinct matter from asking for federal district court review of some particular adverse decision on the merits of a state court case (which is impermissible). In football terms we're not asking for a new set of refs to review a play call we don't like -- we're not allowed to do that (and shouldn't be). We're asking the Competition Committee to correct a defect in the rulebook all the refs are using -- we are allowed to do that (and any thinking person inside the system ought to be thankful when good cases of this sort come up).
