Is it considered usury for a mother to take an amount exceeding what her son completed from his own money to buy household necessities, knowing that the amounts are trivial and the extent of the increase is unknown each time?

What a mother gives to her child is a gift from her, not usury, and there is no harm in taking it. And if it is more than what the child paid, then it is considered a good fulfillment [of a debt], for the Prophet, peace and blessings be be upon him, said: "The best among you are those who are best in fulfilling their obligations."