How can the children of a man who died without paying zakat on his wealth for over twenty years pay it on his behalf from his inheritance, and what is the obligatory amount to be disbursed, knowing that he did not deny the obligation of zakat?

If the wealth mentioned in the question is cash, gold, silver, or trade goods, then his children must determine the amount of wealth he possessed each year. If it reached the nisab (minimum taxable amount), then one-quarter of one-tenth (2.5%) of it must be paid as Zakat. If they cannot ascertain the exact amount with certainty, then it suffices for them to pay what they believe, with a high degree of probability, will discharge the liability, by considering the most cautious amount to absolve the obligation, because negligence in paying Zakat is a grave danger. A hadith from the Prophet, peace be upon him, illustrates the consequence for those who do not pay the Zakat on their wealth, and he recited the verse: "And let not those who hoard what Allah has given them of His bounty think that it is good for them. Rather, it is bad for them. They will be encircled by what they hoarded on the Day of Resurrection." (Aal 'Imran: 180).