Parts to RAP [common-law, pre-USRAP]:
Might it [the Contingent remainder, vested remainder subject to open, or executory interest]
vest [become possessory or class close]
too late [21 years beyond the death of every life in being]
O to A for life; then X; then to X's children.
Since the Vested Remainder subject to Open in FSA will vest upon X's death [or not vest and revert to O] this conveyance does not violate RAP.
If you did:
O to A for life; then to X, then to X's children who reach 30.
You see that after you kill off all the LIBs you /could/ be left with children of X who are under 30 but could reach 30, so it ///could/// remain subject to open 21 years after X's death. And thus violate common-law RAP.