Is it permissible for a father to join both his and his children's cows and give Zakah on them together?
Zakah is not obligatory on the cows that are raised for milk and reproduction unless they reach the Nisab (the minimum amount on which Zakah is due), which is thirty cows or more. The cows of two persons are not to be gathered to complete the Nisab, unless (Part No. 8; Page No. 33) they meet
the condition of joint ownership which the Fuqaha' (Muslim jurists) set, meaning that the cows have a common resting place, pasture, breeding and milking place. If these conditions are met, the joint ownership will be considered as one property. The Prophet (peace be upon him) stated:
Those separated should not be gathered together nor should those gathered together be separated in order to avoid paying Zakah (obligatory charity). Regarding that which belongs to two partners, they can make claims for restitution from each other with equity.
Al-Bukhari in his Sahih [Authentic Hadith Book])May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.