بسم الله الرحمن الرحيم
One of the important conditions for waqf ala al-awlad to be valid is that in case of the absence of the endowed persons (male children here) the benefit of endowment is diverted to the poor; so if the poor were declared the last recipient of the endowment in the question mentioned above, then this waqf ala al-awlad was valid and effective. Thus, in your case, the income of the endowed property will divided among the 9 sons of Hasan and Usman equally i.e. the total income will be divided in 9 shares and each son will get 1 share. If they also have sons i.e. grandsons of Hasan and Usman, then they will also be included in the distribution. For example, if there are 4 grandsons along with the 9 sons, then the total income will be divided in 13 shares and each of them shall get 1 share:
قال في الأشباہ (ص دارالکتب العلمیۃ بیروت): وأما إذا وقف علی أولادہ دخل النسل کلہ کذکر الطبقات الثلاث بلفظ الولد اھ (وانظر رد المحتار 6/697،698 ط مکتبۃ زکریا دیوبند ایضاً)۔
And if the poor were not declared as the last recipients of the endowment, the waqf was not valid and in such case the entire property of the deceased will be inheritance and will be divided among all heirs as per the Shariah.
Allah knows Best!
Darul Uloom Deoband