Showing preference to one child over the others is deplorable and is apt to lead to negative consequences.

A person should never distinguish one of his children from among the other children [with preferential treatment], as due to the extra two-sela weight of the tunic of fine wool that Jacob gave to Joseph above that which he gave to the rest of his sons, Joseph’s brothers became jealous of him, and the episode unfolded and our forefathers descended to Egypt.

With regard to one who wrote a document bequeathing his property to others and left his sons with nothing, what he did is done and the transaction is valid, but the Sages are not pleased with him. (Shabbat 10b; Bava Batra 133b)