Immortality and the sons of Elrond

[Galin]

Didn't Elros already choose his childrens' fate anyway?

I would say both Elros and Elrond's children had the same choice in potential (both are 'potential immortals' for example), but only the children of the immortal need be given the choice.

The mortal (once chosen) will essentially restore the natural order of Eru without sundering child from parent, and the special dispensation given to Earendil and Elwing need not be extended yet again in this circumstance.