The choice of mortality is permanent.
Once you choose the Gift of Men, you are mortal and all your descendants will be mortal. Only if you remain immortal can your descendants have the choice to be mortal or immortal.
In the words of the man himself:
"The view is that the Half-elven have a power of (irrevocable) choice, which may be delayed but not permanently, which kin's fate they will share. Elros chose to be a King and 'longaevus' but mortal, so all his descendants are mortal, and of a specially noble race, but with dwindling longevity: so Aragorn (who, however, has a greater life-span than his contemporaries, double, though not the original Númenórean treble, that of Men). Elrond chose to be among the Elves. His children - with a renewed Elvish strain, since their mother was Celebrían dtr. of Galadriel - have to make their choices."
-- Tolkien, Letter No. 153