Dr. Monzer Kahf, a prominent economist and counselor, states the following: Parents have to equate their children in giving gifts. The Prophet, peace and blessings be upon him, was asked to be a witness on giving to one child and he asked whether the father was giving all his other children equal gift, when the man said no, the prophet, peace and blessings be upon him, said: “I won’t be a witness to injustice.” Calling it injustice means it is haram. Unless giving is related to a specific need, such as giving medicine to the sick child, or taking her on a recuperation trip after long illness, giving must be equal.
However, equality may be in kind or in cash, such as getting one child married, and getting the others also married even if the cost of their weddings are not equal.
We should also remember that boys and girls are equal in gift-giving from their parents since this is not inheritance.”