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

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.