What is the ruling on dealing with conventional banks, and is it permissible to spend on one's children from their money, and is there sin upon one for that, and does the sin of dealing with usury fall upon one who issues a fatwa permitting it, and is it obligatory to transfer money to Islamic banks?

It is not permissible to invest money or deposit it in usurious banks. Usurious gains must be disposed of by spending them on general Muslim welfare or by giving them to the poor and needy. There is no sin upon the child whose father spends on his family from mixed money (containing both lawful and unlawful elements), but the father should be advised to cease dealing with usurious banks. The criterion for Islamic banks is their adherence to Sharia principles, not merely their name. To purify money tainted with unlawful gains, the forbidden interest must be disposed of by spending it on general Muslim welfare. There is no sin in what has been spent unknowingly from it.