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.
First of all, a Muslim is thanked for being keen on knowing the Islamic ruling about what he wants to do before doing it because this is what he is required to do, as there are many negligent Muslims who perform acts of worship or transactions without knowing the ruling of Allaah on them.
Al-Qaraafi said in his book Ath-Thakheerah: “Rule: Anyone who does an act or says a statement or makes a transaction or the like, it is not permissible for him to undertake it until he knows the ruling of Allaah on it. So, if he learns it and acts [according to it], he has obeyed Allaah twice by learning what is obligatory on him to learn and by acting [according to it], if it is an act by which one gets closer to Allaah, otherwise by learning only; and if he did not learn and did not act, then he has disobeyed Allaah twice by abandoning learning what is obligatory on him to learn and by abandoning acting [according to it], if it is an obligatory act, otherwise by abandoning learning only; and if he learns and did not act, then he has obeyed Allaah by learning what is obligatory on him to learn and disobeyed Him by abandoning acting according to it if it is an obligatory act, otherwise no; Ash-Shaafi’i reported a consensus on this rule.”
As regards your question, it is permissible for the father to buy what you mentioned for his son, his daughter and his wife, especially that he equalized between his son and daughter in the amount of gold, as by doing so, one fulfills the obligatory justice that is ordained in the Hadeeth "Fear Allaah and be just between your children." [Al-Bukhari and Muslim]
However, he is not obliged to be just between his children on one hand and his wife on the other hand, but here we will point out to these three matters:
Firstly: If the gift was given in order to avoid paying Zakah, then the Zakah is not waived according to the view of many jurists; this is the view of the Hanbali School: Al-Mardaawi [from the Hanbali School] said in Al-Insaaf: “The correct view of the School is that if what is intended by selling something, gifting it or spoiling it and the like is to escape Zakah, then he is not exempted from paying it; this is the view of the majority of Hanbali scholars and the decisive opinion of most of them, but Abu Ya’la As-Sagheer (the grandson of Al-Qaadhi Abu Ya'la ibn Al-Farraa') stated in his Mufradaat from some scholars of the Hanbali School: “Zakah is waived if one makes a trick to avoid it as per the view of Abu Haneefah and Ash-Shaafi'i.”
According to the view that Zakah is not exempted, then one should pay the Zakah for that year only. The author of Al-Insaaf said: “He should pay the Zakah of the sold item for that lunar year only if he intended to escape Zakah according to the correct view of the School.”
Secondly: The gift of that gold is not effective except when the receiver of the gift possesses it [and disposes of it as its owner]; and the wife possessing it is by her receiving the gold and taking possession of it. But if he bought her that gold and he died before the wife possesses it, then the gift is not effective and the gold remains as inheritance for all the heirs. As regards the children possessing the gold, then it is not a condition for them to take possession of it and own it as long as they are young [below the age of puberty], and the father possessing what is gifted to his young children makes them [the young children] as possessing the gift whether the donor is the father or someone else.
The book of Al-Furooq, Chapter of Gift, reads: “If the father gifts something to his young son and he informs him about it, then this is permissible even if nobody else possesses it on behalf of the young child but if he gifts it to his adult son, then this is not permissible, even if he is included in his children as long as he did not give it to him. The difference is that the father has legal guardianship over his young son, so possessing a gift on his behalf is permissible [and effective]; don’t you know that if a stranger gifts something to a young child, it is his father who takes possession of it, so since it is permissible to take it from a stranger on behalf of his child, then it is also permissible to possess his own gift to the child [i.e. to keep it for him until he becomes adult], but this is not the case in regard to an adult child.”
Moreover, the Fiqh Encyclopedia reads: “The Maaliki and Hanbali Schools are of the view that if the guardian gives something to the interdicted person (like a young child) under his guardianship, and he has witnesses to this, then the gift is effective, and having witnesses is sufficient instead of the interdicted person possessing it. Their evidence for this is what Maalik reported from Az-Zuhri from Ibn Al-Mussayib that Uthmaan said: “If someone gifts his son something but the son is young and not eligible to possess his gift, and the father declared this and had witnesses to this, then it [the gift] is permissible.”
Thirdly: Zakah is not obligatory on that gold after taking possession of it because the share of each one of them (i.e. the wife, the son and the daughter) does not reach the Nisaab (minimum amount liable for Zakah) and it is known that Zakah on gold is not obligatory unless it reaches 85 grams by itself or with other money added to it while other conditions for the obligation of paying Zakah are fulfilled, among which is the expiry of Hawl (i.e. cycle of full lunar year) over possessing the Nisaab, and that, according to the majority of the scholars, the gold or silver should not be kept for adornment; in this regard, please refer to Fatwa 87362.
Therefore, if each one of them does not have money to complete the Nisaab, then they do not have to pay Zakah on it. For more benefit on the Nisaab of gold for which it is an obligation to pay the Zakah, please refer to Fatwa 81332.
Allaah Knows best.