She unknowingly married her daughter to a mushrik, and she has three children from him, and she is asking about the ruling

Praise
be to Allah.

You did not explain what this man has done such that he deserves to be
regarded by you as a mushrik. But whatever the case, if it is in fact proven
that he is a mushrik who ascribes partners to Allah, then it is not
permissible for a Muslim woman to marry him, because Allah, may He be
glorified, has commanded the Muslims not to give their daughters in marriage
to people of shirk, and He has explained the reason for this prohibition,
which is that marriage of a Muslim woman to a mushrik opens the door to her
of confusion about religion and apostasy from Islam. Allah, may He be
glorified, says (interpretation of the meaning):

“And give not (your daughters) in marriage to Al-Mushrikoon
till they believe (in Allah Alone) and verily, a believing slave is better
than a (free) Mushrik (idolater, etc.), even though he pleases you. Those
(Al-Mushrikoon) invite you to the Fire, but Allah invites (you) to Paradise
and Forgiveness by His Leave, and makes His Ayat (proofs, evidences, verses,
lessons, signs, revelations, etc.) clear to mankind that they may remember”

[al-Baqarah 2:221]. 

Al-Baghawi (may Allah have mercy on him) said:

“And
give not (your daughters) in marriage to Al-Mushrikoon till they believe (in
Allah Alone)” – there is consensus on this point: it is not permissible
for a Muslim woman to marry a mushrik man.

“and
verily, a believing slave is better than a (free) Mushrik (idolater, etc.),
even though he pleases you.”

“Those”
i.e., the mushrikeen or polytheists

“invite
you to the Fire” i.e., to deeds that lead one to Hell. 

Al-Qurtubi (may Allah have mercy on him) said in his Tafseer (3/72):

“And
give not (your daughters) in marriage to Al-Mushrikoon” means do not
give a Muslim woman in marriage to a mushrik. The ummah is unanimously
agreed that a mushrik should not marry a believing woman under any
circumstances, because that undermines Islam. End quote. 

Allah, may He be glorified and exalted, says (interpretation of the
meaning):

“O you who believe! When believing women come to you as
emigrants, examine them, Allah knows best as to their Faith, then if you
ascertain that they are true believers, send them not back to the
disbelievers, they are not lawful (wives) for the disbelievers nor are the
disbelievers lawful (husbands) for them”

[al-Mumtahanah 60:10]. 

Ibn Katheer (may Allah have mercy on him) said in his Tafseer (8/93):
“they are not lawful (wives) for the disbelievers nor are the
disbelievers lawful (husbands) for them”. This verse is the one which
forbade Muslim women as wives for mushrik men. At the beginning of Islam it
had been permissible for a mushrik man to marry a believing woman. End
quote. 

Ash-Shawkaani said in Fath al-Qadeer (5/256): The sentence “they
are not lawful (wives) for the disbelievers nor are the disbelievers lawful
(husbands) for them” explained the reason why they should not be turned
back. This indicates that a believing woman is not permissible for a
disbelieving man, and that if a woman becomes Muslim, she must be separated
from her (mushrik) husband. End quote. 

You should have been careful with your daughter and chosen from her
potential suitors one who was religiously-committed and of good character,
and you should have enquired about him and found out more about him. But you
were careless and heedless about that, to the extent that this happened and
she married that man who is a mushrik, according to what you say. This
marriage was undoubtedly invalid from the moment it occurred, according to
Muslim consensus, so what your daughter must do is leave this man
immediately, without talaaq (divorce), because talaaq can only result from a
valid marriage, but this marriage was invalid from the outset. 

Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him) was asked: A
man married a Muslim woman, then it became clear that the man was a kaafir.
What is the ruling? 

He
replied:

if
it is proven that the man mentioned was a kaafir at the time of the marriage
contract, and the woman was a Muslim, then the marriage is invalid, because
it is not permissible, according to Muslim consensus, for a kaafir to marry
a Muslim woman, because Allah, may He be glorified, says (interpretation of
the meaning):

“And give not (your daughters) in marriage to Al-Mushrikoon
till they believe (in Allah Alone)”

[al-Baqarah 2:221].

“… then if you ascertain that they are true believers, send
them not back to the disbelievers, they are not lawful (wives) for the
disbelievers nor are the disbelievers lawful (husbands) for them”

[al-Mumtahanah 60:10]. 

End quote from Fataawa Islamiyyah (3/230) 

In
a statement of the Islamic Fiqh Council it says: 

Marriage of a kaafir to a Muslim woman is haraam and is not permissible,
according to scholarly consensus, and there is no doubt about that because
of what is stated in the shar‘i texts. 

End quote from Fataawa Islamiyyah (3/231)

What you must do is repent to Allah, may He be glorified, from this
heedlessness, and you should hasten to rectify the sitution with regard to
your daughter. 

All of this applies if what you meant by saying that this man is a mushrik
is that it became clear to you that he follows a religion other than Islam,
such as the Babi, Baha’i, Qadiani, Bohra and other sects that are outside
the religion of Islam. 

But if what you meant is that he is a Muslim, in basic terms, but he does
some of the acts of the mushrikeen that are not obvious to Muslims like him,
especially in countries where knowledge of Islam and the Sunnah is weak,
then in this case he should be told about what the Messenger taught about
such matters, and the religion of Allah should be explained to him, and
proof from the Sunnah should be established for him. Then if he repents from
his ways and gives up, there is nothing wrong with your daughter remaining
married to him. 

But if he insists on following that way, then the ruling is as described
above, that she must separate from him. 

You should understand that according to Islam, it is essential to have a
wali (guardian) for marriage, and it is not permissible for a woman to give
herself or another woman in marriage. This has been explained in detail in
fatwa no. 104852 

And Allah knows best.