All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
A father is obliged to be just between his children when giving them gifts as we have already clarified in Fatwa 123876.
However, if some children help their father with a particular project by working and putting in efforts that the others do not, then the father may give the one who does the work a salary commensurate with that of a worker who does the same job. Alternatively, he may determine a fixed salary for him which should be within the prevalent norms and what is customary in return for his efforts. In case the salary goes beyond the norms in return for the effort that he puts in, then the father would be unduly favoring him and it is forbidden to do so between one's children, as clarified in the Fatwa referred to above.
Therefore, the Islamic ruling on giving gifts to the children should be clarified to the father in a soft and gentle manner.
Finally, it should be noted that it is an obligation to be kind and dutiful to the father and maintain good ties with him in every condition. Even if the father is in the wrong and misbehaves, it is not a sound reason to mistreat him, otherwise this is considered as severing ties with him.
For more benefit, please refer to Fataawa 84942 and 82254.
Allaah Knows best.