What is the ruling on developing and trading with surplus funds from endowments designated for offspring, and what is the ruling on taking a percentage for developing them?

The surplus money from the endowment's proceeds is not an endowment itself. Rather, it is a right belonging to the beneficiaries of the endowment and must be paid to them. To develop this money, permission must be obtained from the adults among them, and from the guardian (wali) or custodian (wasi) for those who are not adults. Permissible forms of development include Mudarabah (profit-sharing) with a percentage of the profit. As for the supervisor (nazir) taking something in return for developing the money, if the endower (waqif) specified a payment for him, then it is permissible for him to take it. Otherwise, he is not entitled to anything unless he is poor, in which case he may take a reasonable amount (bil-ma'ruf), or if he is known for taking the customary wage for his work, then he may do so.