Short answer. One-half of the estate is reserved for the illegitimate children as their legitime, and they divide that half among themselves in equal shares. The other half is free portion, which your father could leave by will to anyone he chose, including to the same children.
What the law says
When the testator dies leaving illegitimate children and no other compulsory heirs, such illegitimate children shall have a right to one-half of the hereditary estate of the deceased. The other half shall be at the free disposal of the testator.
Civil Code, Article 901 — Illegitimate Children Alone. Read the full provision →
What "no other compulsory heirs" means
The half-share applies only where the illegitimate children stand alone. Compulsory heirs are the class the law refuses to let a testator disinherit without cause: legitimate children and their descendants, in their absence the legitimate parents and ascendants, the surviving spouse, and illegitimate children. If any of these also survived your father — a legitimate child, a widow, or his own surviving parents — this article is not the one that governs, and the illegitimate children's share is computed under the rule matching that combination instead. The first question in any estate is therefore not arithmetic but a headcount: who was alive on the date of death, and in what relationship to the deceased.
How this article reads today
The fraction was worked out on the classification of children used when the Civil Code took effect in 1950, and the article must be read subject to the Family Code. The Family Code provides that the legitime of each illegitimate child is one-half of the legitime of a legitimate child, and that "except for this modification, all other provisions in the Civil Code governing successional rights shall remain in force." The Family Code also recognises a single class of illegitimate child, so the older Civil Code sub-classifications — natural, acknowledged natural, and the rest — no longer place children in different tiers. Where illegitimate children are the only compulsory heirs, the reserved half stands and is divided equally among them.
The free half, and what it can and cannot do
The other half is genuinely free: your father could have left it to a friend, a charity, a partner, or to the same children on top of their legitime. But the freedom runs only to the free half. A will that gives the whole estate away and leaves the illegitimate children nothing impairs their legitime and can be reduced to that extent. Donations made during his lifetime are not automatically safe either — gifts that are in truth advances on inheritance are brought into the computation. If he left no will at all, this article's split does not apply; the estate passes by intestate succession, and the children may take more than half.
Proving the claim
Everything turns on proving filiation, and it is where these estates are usually won or lost. The strongest evidence is a birth certificate the parent signed or participated in, a written acknowledgment, an admission of parentage in a public or private document, or open and continuous possession of the status of a child. A father's name entered on a birth record without his participation carries little weight on its own. Assemble civil registry copies, school and medical records naming him, and correspondence, before the estate is settled. Do not sign a settlement or waiver until the shares are computed. If the other heirs deny your filiation, book a consultation.