You can open bank accounts for each of your children, but on the condition that you treat them fairly, and do not prefer any of the males over the others, or any of the females over the others. So each male should have the share of two females. This is an important matter which must be paid attention to. If you need any of this money, then you may take whatever you want, but you must make sure that you are fair so that the amounts left are fair, and that the males have twice the share of the females, and you do not show preference to one child over another.
You must also pay attention to zakaah each time a year passes. Try to find a bank that does not deal in ribaa (if there is one) so that you will not be cooperating in disobeying Allaah. We ask Allaah to give you strength.