Short answer. Yes. Article 866 of the Civil Code provides that the second heir acquires the right to the succession from the time of the testator's death, even if the second heir dies before the fiduciary. That right passes to the second heir's own heirs — your children inherit your position in the substitution.
What the law says
The second heir shall acquire a right to the succession from the time of the testator's death, even though he should die before the fiduciary. The right of the second heir shall pass to his heirs.
Civil Code, Article 866 — The Second Heir's Right Vests at Death. Read the full provision →
The right vests at the testator's death, not later
Article 866 of the Civil Code resolves a critical timing question in fideicommissary substitutions: when does the second heir's right actually arise? The article answers clearly: The second heir shall acquire a right to the succession from the time of the testator's death, even though he should die before the fiduciary. The second heir does not have to survive until the fiduciary actually hands over the property. The right comes into existence the moment the testator dies — even before the fiduciary has had possession, and even if the second heir dies during the fiduciary's holding period.
The right passes to the second heir's own heirs
Because the right vests at the testator's death, it becomes part of the second heir's own estate. Article 866 follows through on this logic: The right of the second heir shall pass to his heirs. If you were named as the second heir (the fideicommissary) and you die before the fiduciary transfers the property to you, your own heirs step into your position. They become entitled to receive the property when the time for transfer arrives — not because they were named in the will, but because they inherited your vested right from your estate.
Why this rule matters in a fideicommissary substitution
In a fideicommissary substitution, the testator names a first heir (the fiduciary) who holds and enjoys the property during their lifetime, with the obligation to preserve it and eventually transmit it to the second heir (the fideicommissary). There can be a long gap between the testator's death and the moment when the transfer from the fiduciary to the second heir actually happens. Without Article 866, a second heir who died during that gap would lose the right entirely, and the property might revert or pass differently. Article 866 ensures that the testator's intention — to ultimately benefit the second heir's family line — is not defeated by the accident of the second heir dying before the fiduciary.
Practical implications for your heirs
For your heirs, Article 866 means that they are entitled to what you were entitled to receive. When the fiduciary's obligation to hand over the property falls due, your heirs step into your place as the persons to whom the transfer must be made. They do not need to be named in the original will to claim this right — they claim it through inheritance from you. To exercise this right properly, they would need to establish their status as your heirs and notify the fiduciary and the estate that the second heir's right has passed to them. The right is transmissible, not personal to the named second heir.