If one’s parents give the child money to spend as he wishes, can one use it for tzedakah, even if had they known they would have said that he shouldn’t / doesn’t need to give tzedakah as frequently as he does?
It is fine to give one tenth to tzedakah even when one knows that the person giving the money would not approve of this.
If the person giving the money gives an explicit instruction that no part of the money should be given to tzedakah, the instruction should be heeded, and no part of the money should be given to tzedakah (Be’orach Tzedakah 2:28, citing from Rav Nissim Karelitz; see also Iggros Moshe, Yoreh De’ah 2:112).
Be’orach Tzedakah cites (at the end of the book, teshuvah 65) from Rav Chaim Kanievsky that when the person giving the money stipulates that maaser should not be given, one should refrain from accepting the money. However, there is no formal prohibition in accepting the money, and this ruling is more by way of recommendation, to avoid the problem.
If one receives money from other sources, one should give maaser from the money, even against one’s parents instructions.