You should treat your daughters and sons equally.

Some people show favouritism in the way they give
gifts to some of their children but not others.
According to the soundest opinion, doing this is
haraam unless there is some legitimate reason,
such as one child being in greater need than the
others due to sickness, being in debt, being
rewarded for having memorized the entire Qur’aan,
being unable to find a job, having a large family,
being a full-time student, etc.
When a father gives something to one of his
children who is in need, he should have the
intention to give similarly to any other children of
his should the need arise.
General proof of this principle may be seen in the
aayah(interpretation of the meaning):
“. . . Be just: that is nearer to piety, and fear
Allaah . . .”
[al-Maa’idah 5:8].
Specific evidence is found in the report of al-
Nu’maan ibn Basheer (may Allaah be pleased with
him), whose father brought him to the Messenger
of Allaah (peace and blessings of Allaah be upon
him) and said:
“I have given this son of mine a slave that I had.”
The Messenger of Allaah (peace and blessings of
Allaah be upon him) asked him,
“Did you give all your children a similar gift?”
He said, “No.”
The Prophet (peace and blessings of Allaah be
upon him) said:
“Then take (the gift) back.”
(Reported by al-Bukhaari, see al-Fath, 5/211).
According to another report, the Prophet (peace
and blessings of Allaah be upon him) said:
“Fear Allaah and be fair to your children,”
so he went away and took his gift back.
(al-Fath 5/211).
According to a third report, he said,
“Do not ask me to be a witness, for I will not bear
witness to injustice.”
(Saheeh Muslim, 3/1243).
Sons may be given twice as much as daughters, as
in the case of inheritance, according to Imaam
Ahmad, may Allaah have mercy on him (Abu
Dawud, Masaa’il al-Imaam Ahmad, 204; Imaam Ibn
al-Qayyim explained this in more detail in his
footnotes).
But there are some families where fathers do not
fear Allaah, and their favouritism creates hatred
and jealousy among the children.
A father may give one child more because he looks
like his paternal uncles, and give less to another
child because he looks like his maternal uncles; or
he may give more to the children of one wife and
less to the children of another wife; or he may put
one wife’s children in private schools, but not the
other wife’s children.
This will ultimately backfire on him, because in
most cases the one who is deprived in this way will
not respect his father or treat him kindly in the
future.
The Prophet (peace and blessings of Allaah be
upon him) asked the man who showed favouritism
to one son,
“Do you not want all your children to respect you
equally?”
(Reported by Imaam Ahmad, 4/269; see also
Saheeh al-Jaami, 1623).
==========================
Can sons be given twice as much as daughters
generally? Or is it just for inheritance?
Answer:
Fair treatment between males and females means
treating them fairly as enjoined by ALLAH in the
verse (interpretation of the meaning):
"ALLAH commands you as regards your children's
(inheritance): to the male, a portion equal to that
of two females."
[Al-Nisa' 4:11]
So if you give the male two riyals, give the female
one.
But fairness in spending means giving each one
what he needs.
A female may need clothes worth two hundred
riyals whilst the male needs a hat worth ten riyals.
Whatever the case, give him what he needs and
give her what she needs,such as if one of them
needs to get married, then arrange his marriage
and do not give the others something of equal
value,except those who have reached the age of
marriage - arrange their marriages too. This is the
issue of spending and maintenance.
Fairness in this case means giving each one what
he needs.
As for giving voluntary gifts,fairness means giving
the male twice what you give to the female, as
stated above, as ALLAH(Subhana Wa Ta'la) has
decreed their shares in inheritance to be.
[Shaykh Ibn 'Uthaymeen(RadiAllahu Anhu)
Liqaa'aat al-Baab al-Maftooh (2/63)
(Thanks to sister Ukht Sauda for introducing this
article to us.)