He wants to give a piece of land to his children, sons and daughters; how should he divide it among them?

Praise be to Allah.

It is obligatory to be fair and just when giving to one’s children, because of the report narrated by al-Bukhaari (2587) and Muslim (1623) from al-Nu‘maan ibn Basheer (may Allah be pleased with him), that the Prophet (blessings and peace of Allah be upon him) said: “Fear Allaah and treat your children fairly.” 

The scholars of the Standing Committee for Issuing Fatwas said: 

It is not permissible for parents to differentiate between the children in giving gifts, because the Prophet (blessings and peace of Allah be upon him) said: “Fear Allaah and treat your children fairly”, and because that causes envy, hatred, resentment and grudges, and severs ties between siblings, and all of that is contrary to the aims of sharee‘ah which came to encourage harmony, close ties, friendship and warm feelings between relatives and family members. End quote. 

Fataawa al-Lajnah al-Daa’imah, 16/225 

If the children include both males and females, then treating them fairly and justly means giving each male the share of two females, as in the case of dividing inheritance, because there is nothing more just and fair than the division ordained by Allah, may He be exalted. 

The scholars of the Standing Committee for Issuing Fatwas said: 

Whoever gives gifts to his children when still alive must treat them all fairly and justly, so he should give the male the share of two females, as in the way of dividing inheritance that is enjoined by Allah, may He be exalted, and because ‘Ata’ said: They used not to divide it except in accordance with the book of Allah, may He be exalted. End quote. 

Fataawa al-Lajnah al-Daa’imah, 16/213 

Shaykh Saalih al-Fawzaan said: 

What is required of the father is to treat his children fairly and justly when giving, so he should give each male the share of two females, following the way of division ordained by Allah, may He be glorified and exalted. End quote. 

Al-Muntaqa min Fataawa al-Fawzaan, 87/14 

Based on that, your grandfather should divide the land between his children, giving each male the share of two females, and in doing so he should pay attention to its value. 

If his children willingly agree to give some of them more than others, there is nothing wrong with that. 

And Allah knows best.