Undoubtedly sponsorship of orphans is one of the greatest righteous deeds, and it is something good and praiseworthy that Allaah has enjoined in His Book, and that our Prophet (peace and blessings of Allaah be upon him) has encouraged in his Sunnah, as has been explained in the answer to question no. 47061. 

It is permissible for the heirs to allocate part of the estate for this purpose, so long as they are adults of sound mind. As for a minor, it is not permissible to donate any of his wealth even if he gives permission, because his permission does not count. If there are minors among the heirs, it is permissible only for the adults of sound mind to donate some of their own shares for sponsoring orphans. 

If someone donates some of the minor’s wealth, then he must pay it back to the minor from his own wealth. 

It says in Sharh Muntaha al-Iraadaat (2/175): It is haraam for the guardian of a minor to dispose of anything except in the interests of his ward, because Allaah says (interpretation of the meaning): “And come not near to the orphan’s property, except to improve it” [al-An’aam 6:152]. If the guardian gives charity or a gift, or indulges in favouritism by selling some of his ward’s property for less than it is worth, or he buys something with it for more than it is worth, then he is liable for what he donated or showed favouritism in. End quote. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked about the heirs giving up a piece of land to one of their brothers so that he could build a house on it. 

He replied: If the heirs are all adults of sound mind, there is nothing wrong with them giving up a piece of land to one of their brothers. But if there are any minors among them, it is not permissible for any of them to give up what belongs to these minors, i.e., nothing should be taken from their share of the estate. But if one of the adults of sound mind gives up his share to his brother, there is nothing wrong with that. End quote from Fataawa Noor ‘ala al-Darb. 

And Allaah knows best.