If this client paid you to take care of his child so he could work, then provide the Tax ID so he can enter it in order to get the childcare credit on his tax return. Many childcare providers get Tax ID's so that they will not have to give out their Social Security numbers. And...of course, the amount he enters on his return for the credit will have to match the amount of income you enter from that childcare when you prepare your own tax return. The IRS cross checks amounts from parents and childcare providers.