What is the ruling on parents' haphazard spending on their adult children, and is it permissible to favor one of them over another?

If parents spend on their children according to need, taking into account what suits each individual, then there is no sin upon them for differential spending. As for gifts, the majority of scholars are of the opinion that it is desirable, not obligatory, to equalize among children. However, some scholars have said that favoritism is forbidden unless there is a valid reason for it, such as a child's illness, large family, or pursuit of knowledge, and this is the more preponderant opinion. If favoritism in giving gifts is without a reason, then it is forbidden, an injustice, and a cause for disobedience and hatred. Parents should be advised gently and wisely. However, if the favoritism is for a valid reason, then there is no harm in it.