Short answer. Yes, accretion can happen between a child and a stranger. Article 1021 of the Civil Code provides that among compulsory heirs, the right of accretion applies when the free portion is left to two or more of them, or to any one of them together with a stranger, not only when it is left among the children alone.
What the law says
Among the compulsory heirs the right of accretion shall take place only when the free portion is left to two or more of them, or to any one of them and to a stranger.
Civil Code, Article 1021 — Accretion Among Compulsory Heirs. Read the full provision →
What the law says
Should the part repudiated be the legitime, the other co-heirs shall succeed to it in their own right, and not by the right of accretion.
Civil Code, Article 1021 — Accretion Among Compulsory Heirs. Read the full provision →
Accretion covers a child paired with a stranger, not just child with child
Article 1021 gives two separate situations where accretion applies among compulsory heirs, and your case fits the second one: among the compulsory heirs the right of accretion shall take place only when the free portion is left to two or more of them, or to any one of them and to a stranger. So it is not limited to shares distributed only among the children. A share left to one child together with an outside stranger also qualifies.
This rule is specifically about the free portion
The article ties this right to accretion to the free portion of the estate, the part your father was free to dispose of by will beyond what the law reserves for compulsory heirs. When that free portion is what was left jointly to a child and a stranger, and one of them cannot or will not take their share, accretion under this article is what governs whether the other gets to absorb it.
The legitime does not work the same way
Article 1021 draws a line between the free portion and the legitime: should the part repudiated be the legitime, the other co-heirs shall succeed to it in their own right, and not by the right of accretion. If what was repudiated is part of the legitime rather than the free portion, the surviving co-heirs take it because the law itself gives it to them, not because accretion transferred it.
What this means for your father's will
If your father left part of the free portion jointly to one of his children and to a stranger, Article 1021 allows accretion to operate between them just as it would between two children who were left a share together. Whether accretion actually applies in a given situation still depends on the details of how that share was left and whether one of the two parties actually failed to take it.