What is the ruling on a father setting aside money for his minor children by depositing it into separate bank accounts for each of them, with the intention that they own it, and is it permissible for him to use this money later for daily family expenses?

You can open a bank account for each child, observing fairness between them, with the male receiving the share of two females. If you need some money, you can take from it, observing fairness and distribution, with the male receiving the share of two females. The zakat (definition) on this money must be observed every year, and it should be deposited in a bank that does not deal with usury, if possible.