Aruch HaShulchan, Laws of Tzedakah, 251:9

Our rabbi the Beit Yosef (R. Yosef Karo) wrote that we force a parent to provide sustenance to their poor child, even if the child is an adult (and the parent is not legally obligated to support them.) And we force the parent more strongly than we force the rest of the wealthy of the city (to support the poor) - until here are his words. And he derives this principle from the words of the Rashba, who clarifies that this regards a wealthy parent who contributes to the town’s tzedakah fund, and instructs that money from this donation shall go towards supporting their poor child. In this case we do not listen to the parent, and we force the parent to support the child from the parents’ own money and not the tzedakah fund. And we also learn from this example that we also provide a little for the child from the tzedakah fund as well, but the majority of the child’s sustenance must come from the parents. There is proof for this from the gemara in Baba Batra and the Mordeichai, who wrote that a wealthy person is obligated to support their poor relatives, and not through the collector of the communal fund. And it is clear that (one who supports their poor relatives) can consider this a portion of their obligatory donation of 10% of their earnings to the community, but one must also donate to the rest of the poor as well. [Translation by AJWS]