Praise be to Allah.
It is obligatory for the father to give zakat al-fitr on behalf of himself and on behalf of those on whose maintenance he spends, namely his children and his wife. If any of his children has his own wealth, that child must give zakat al-fitr on behalf of himself.
If your father did not know that zakat al-fitr is obligatory, then we hope that he will not be held to account for that, but he must give it on behalf of himself and those on whom he spends on for the past years.
It says in Mawaahib al-Jaleel (2/376):
It says in al-Mudawwanah: If the one who can afford it delays it, he must make it up for the past years. End quote.
It says in Mukhtasar al-Waqaar: If someone neglects it for several years, and he can afford it, he must give it for the years that he neglected, on behalf of himself and on behalf of everyone on whose behalf he was obliged to give it every year, giving the amount that he was required to give, even if that consumes all his wealth, if he is in good health. If he is sick, and leaves instructions to that effect, it may be taken from one third of his estate. End quote.
If you volunteer to give it on your own behalf for the years during which your father was spending on your maintenance, then you should inform him of that, and there is nothing wrong with doing it, although the basic principle is that it is an obligation that he owes.
It is permissible to give the zakat al-fitr of these years to one person or to a number of people, and you do not have to give it to twenty-five individuals, because what is required is to give it to a poor person, and so long as someone meets this description, it is permissible to give him more than one fitrah, from the same giver, or from a number of givers, so long as his poverty is not cancelled by what he takes.
It says in al-Mughni (3/99): It is permissible to give to one person what a number of people are obliged to give, and to give to a number of people what one person is obliged to give.
With regard to giving to a number of people what one person is obliged to give, we do not know of any difference of scholarly opinion concerning that, because he is giving his charity to those who are entitled to it, so he has discharged his obligation, in the same way as if he gave it to one person.
As for giving one person the charity of a number of people, ash-Shaafa‘i and those who agreed with him said that it is obligatory to distribute the charity to six categories of recipients, and to give the share of each category to three people who come under that heading, as we have noted above.
We have quoted the evidence for that, and because it is charity that is not given to a specific individual, it is permissible to give it to one person, as in the case of voluntary charity.
This was the view of Maalik, Abu Thawr, Ibn al-Mundhir, and ashaab ar-ra’y. End quote.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: Zakaat al-fitr is one saa‘ (of food) for each person, but there is no specific number of poor people to whom one may give it.
Hence it is permissible to share out the fitrah among more than one poor person, and it is permissible to give a number of fitrahs to one poor person." (ash-Sharh al-Mumti‘ 15/161).
Zakaat al-fitr is to be given in the form of food, such as wheat and rice. The weight of one saa‘ of these foods varies. If it is given in the form of flour, the saa‘ is two kilograms, so you must give fifty kilograms of flour.
If it is given in the form of rice, the saa‘ is approximately three kilograms, so you must give seventy-five kilograms of rice.
And Allah knows best.