What should one do who possesses a sum of money and wishes to save it for their children in an Islamic bank, knowing that they are an employee and do not engage in any commercial activity?

There is no objection to depositing money in Islamic banks as an investment deposit to develop it and avoid the decrease of capital due to zakat, as zakat becomes obligatory on money if it reaches the nisab and a full year has passed on it. It is preferable to develop it through reliable Islamic banks or legitimate Mudarabah with trustworthy individuals.