Sorry but you are misinformed. The increased child tax credit passed in the House of Representatives and is now being debated in the Senate. It is not a law yet. When and if it becomes a law we will find out how and when it will be implemented.
While this has been in discussions, the full Congress has not yet enacted an increased CTC. Nothing can happen until it does, the President signs the bill, and the IRS implements and communicates procedures for handling it. It might not even involve the IRS, and we don't know yet if it is even retroactive to 2020.