This is a duty upon every sane Muslim, who has the value of Nisab. Fathers are instructed to give Fitr on behalf of those children who are not of age yet.
The amount that must be given is equal to 1.6 kg of wheat or 3.2 kg of barley = which equates to £3. This does not mean that a person must distribute wheat or barley, one may give its equivalent value.
Fitr is an emphasized Sunnah which becomes due before ‘Eid Salah, although it is preferable to give it a few days before ‘Eid so that the poor actually receive it and are able to spend it on `Eid day. If one does not give the Fitr, it will remain due no matter how much time passes after ‘Eid. This will become Sadaqah not Fitr.