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, you should know that it is the grandfather or the Wasiyy (i.e. the executor of the will of the deceased father) are given priority to take care of the property of the orphan and his affairs, and the scholars differed in opinion on who is the most entitled to do so among the two.
If the orphan’s father had not made a will and the orphan has no grandfather, then it is the judge or whoever acts on his behalf who would take care of his (the orphan’s) affairs or that he appoints someone else who is eligible for doing so.
Hence, if you have guardianship over these children or that a Muslim judge put their matter in your hands, then you are Islamically permitted to take from their money for yourself according to what is reasonable as Allaah Says (what means): {And test the orphans [in their abilities] until they reach marriageable age. Then if you perceive in them sound judgment, release their property to them. And do not consume it excessively and quickly, [anticipating] that they will grow up. And whoever, [when acting as guardian], is self-sufficient should refrain [from taking a fee]; and whoever is poor — let him take according to what is acceptable. Then when you release their property to them, bring witnesses upon them. And sufficient is Allaah as Accountant.}[Quran 4:6]
However, the jurists had extensively differed about what is meant by taking (from the property of the orphans) according to what is reasonable to a point that their statements reached seven different ones. Some of them were very strict and forbade taking from it and they interpreted taking according to what is reasonable here to being restricted to what is sufficient to fulfill one’s necessity only so that one would not need to consume the property of the orphan (with no right). While some others permitted for one to take from the property of the orphan and said that he is not obliged to give back what he took when he will be well-off. Some others said that it is an obligation to give back what he took. Indeed, this last statement is the most balanced among all these statements and it is better to act according to it in order to be on the safe side.
Al-Qurtubi said in his Tafseer (interpretation of the Quran): “The majority of the scholars differed in opinion in regard to the meaning of taking (from the property of the orphan) according to what is reasonable; some said: “It is a loan if one is in need and he should pay it back when he will be solvent.” This is said by ‘Umar Ibn Al-Khattaab, Ibn ‘Abbaas, ‘Ubaydah, Ibn Jubayr and Ash-Sha’bi .
According to this statement, borrowing from their money, if it was for a need, is permissible on the condition that you give it back when you will be well-off.
Of course, all this is based on your being an executor of a Sharee’ah-based will of the deceased father or being appointed as a guardian by the judge. However, if you just disposed of their money without any valid reason, then you are obliged to repent to Allaah and hasten to give back the money.
Indeed, the Sharee’ah is very firm in handling the property of the orphans and threatened the one who consumes it without right with a severe threat. Allaah Says (what means): {Indeed, those who devour the property of orphans unjustly are only consuming into their bellies fire. And they will be burned in a Blaze [i.e. Hellfire].}[Quran 4:10]
Allaah Knows best.