Praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
It is permissible to favor a child over another in gift giving for a special reason or need. Therefore, it is permissible to help the sick child get treatment without having to compensate the rest of the children and the same applies to paying a child`s tuition fees, and the like.
The proof of this is that Abu Baker (May Allah have mercy on him) favored Ayisha (may Allah be pleased with her), the Mother of the believers and the wife of the Prophet (Peace be upon him), over her sisters where he gifted her an orchard to suffice for her needs.
However, the Prophet (Peace be upon him) forbade favoring a child over another in gift giving without a valid excuse. This is reflected in the following Hadith: "Nu'man b. Bashir reported that his mother bint Rawaha asked his (Nu'man's) father about donating some gifts from his property to his son. He deferred the matter by one year, and then set forth to do that. She (Nu'man's mother) said: I shall not be pleased unless you call Allah's Messenger (Peace be upon him) as witness to what you confer as a gift on your son. (Nu'man said): So father took hold of my hand and I was at that time a boy, and came to Allah's Messenger (Peace be upon him). and said: Allah's Messenger, the mother of this son (of mine), daughter of Rawaha wishes that I should call you witness to what I confer as gift to her son. Allah's Messenger (may Allah be pleased be upon him) said: Bashir, have you any other son besides this (son of yours)? He said: Yes. He (the Holy Prophet) said: Have you given gifts to all of them like this? He said: No. Thereupon he (the Holy Prophet) said: Then call me not as witness, for I cannot be witness to an injustice." {Related by Muslim}.
However, if male children needed decent accommodation while the females didn`t, then it is permissible for the father to favor each son with a piece of land and he isn`t considered sinful for doing so. Yet, we advise him to explain this ruling to his daughters to leave no room for grudges or feelings of injustice. And Allah The Almighty Knows Best.