A person died but had made sacrifice on his behalf while he was still alive, his children then performed pilgrimage on his behalf after his death. Ever since he died his children have been sacrificing and giving charity on his behalf. What is the Islamic ruling on sacrifice? is it permissible or it is regarded as charity? الأضحية عن الميت

In the name of Allah, The Most Gracious, The Most Merciful.

Praise be to Allah, and may Allah’s peace and blessings be upon the Messenger of Allah, his family and his companions.

To proceed:

It is more befitting for anyone who wants to do good on behalf of his deceased relatives and confer reward upon them to strive in supplicating for them because it is indeed the act with best rewards that can reach them after death. In supplication there is benefit for the deceased and the ones alive because the person supplicating is rewarded for his supplication and it is said to him,  "May you receive the like." However, the only ones who will benefit from the rewards of acts that are not supplication are the ones that would have been conferred upon the rewards for those acts. As for sacrificing, it is ordained for the living. The deceased are consequently included in the sacrifice done by the living, but if the deceased had ordered in a will that sacrifice should be offered from the wealth left after death then that order should be executed from that wealth.
And Allah knows best.