Am I permitted to take back my own money from my husband without his knowledge, after he seized it, given that he is remiss in spending on the household and children?

If the husband does not stipulate taking a portion of his wife's salary in exchange for allowing her to work, then her salary is her sole right, and it is not permissible for him to take anything from it without her consent. If the husband takes from his wife's money unlawfully, he is an oppressor, and it is permissible for the wife to take from his money an amount equal to what he took from her, without his knowledge. If she takes more than that, and it is equal to her and her children's maintenance according to what is customary, then it is permissible, as evidenced by the hadith (definition) of Hind bint Utbah. However, if it exceeds the customary maintenance, it is not permissible, and she must return the excess. If she is in doubt about the excess amount, she must return what she believes is likely the amount of the excess.