In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
It is allowed for a woman to take money from her husband without his knowledge to meet her and her children’s needs if he is not giving them what they are customarily entitled to.
The late Saudi scholar, Sheikh Ibn Baz (may Allah have mercy on him) stated:
She must take the money without extravagance or waste if he does not give her what suffices her. This is based on what is recorded in Sahih al-Bukhari and Sahih Muslim from `A’ishah who stated that Hind bint `Utbah came to the Prophet (peace be upon him) and said, “O Messenger of Allah (peace be upon him), Abu Sufyan does not give me what is sufficient for me or for my children.” The Prophet (peace be upon him) said, “Take from his wealth according to what is right and good which will suffice you and your child.”
Almighty Allah knows best.
Source: Islamic Fatawa Regarding Women, Compiled by Muhammad Ibn Abdul-Aziz Al-Musnad and translated by Jamal Al-Din Zarabozo, Darussalam, 1996.