Praise be to Allah.
If your husband is spending on you and on his children, then it is not permissible for you to take any of his money, because it is haraam to take money except with the consent of its owner. Allah, may He be exalted, says (interpretation of the meaning):
“O you who believe! Eat not up your property among yourselves unjustly except it be a trade amongst you, by mutual consent”
And the Prophet (blessings and peace of Allah be upon him) said: “Verily your blood, your wealth and your honour are sacred to one another, as sacred as this day of yours in this month of yours in this land of yours. Let those who are present convey it to those who are absent.” Narrated by al-Bukhaari (67) and Muslim (1679).
And he (blessings and peace of Allah be upon him) said: “It is not permissible to take the wealth of a Muslim unless he gives it willingly.” Narrated by Ahmad (20172); classed as saheeh by al-Albaani in Irwa’ al-Ghaleel (1459).
If he falls short in obligatory spending, it is permissible for you to take that from his wealth on a reasonable basis, because of the hadith of ‘Aa’ishah (may Allah be pleased with her) according to which Hind bint ‘Utbah said: O Messenger of Allah, Abu Sufyaan is a stingy man and he does not give me what is sufficient for me and my children, except what I take from him without him knowing. He said: “Take what is sufficient for you and your child on a reasonable basis.” Narrated by al-Bukhaari (5364).
But if he is not falling short in obligatory spending, then it is not permissible for you to take anything of his wealth without his consent.
So beware of taking from his wealth that which is not permissible for you, or of hiding anything of it, even if that is on the grounds of saving it for his children, because you have no authority to do that and the children have no right to their father’s wealth whilst he is still alive, apart from what is needed for their maintenance, unless your husband gives you permission to save (some of his money), in which case there is nothing wrong with doing so.
So if you tell him that you are going to save any surplus from household costs for the children, for example, and he gives permission to do that, then there is nothing wrong with it, and it will come under the heading of a conditional gift to be given when one has the means to do so.
You should also advise your husband to fear Allah, may He be exalted, to remember that He is always watching, and to take care of his wealth.
You should also be wise in calling him to goodness and diverting him from evil; you should be patient and seek reward with Allah, and focus on raising your children. You should be patient in bearing what you dislike of his conduct, for that is better for you than destroying the household and causing hardship to the children.
The Messenger of Allah (blessings and peace of Allah be upon him) said: “Know that there is a great deal of good in patiently bearing what is disliked, victory comes with patience and relief comes with distress and with hardship comes ease” Narrated by Ahmad (2803) and others, from the hadith of Ibn ‘Abbaas. Classed as saheeh by Shaykh Ahmad Shaakir and the commentators on al-Musnad.
Please see also the answer to question no. 154172, in which we described some wise methods that the wife may follow in calling her husband.
We ask Allah to guide your husband and put your mind at rest.
And Allah knows best.