بسم الله الرحمن الرحيم
In the abovementioned case if your student’s father has income sources other than the income you mentioned then you should tell him that he must give you the salary from halal income or by borrowing money from someone. If he is not ready for the same then you must not teach his children on fee, rather look for someone else where you could get halal salary.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.