First of all, let me clear up a misconception: It is not illegal, immoral, or fraudulent to put whatever numbers you need to on your W-4 to get the right amount of tax withheld from your paycheck, even if those numbers don’t necessarily match reality. The W-4 only goes to your employer, not the IRS, and it is only used to calculate withholding. Think of the W-4 as just a worksheet to help you and your employer figure out how much to withhold. Yes, you can get in trouble if you have too little withheld, but it’s the fact that you withheld too little that’s the problem, not the fact that you wrote down the wrong number of dependents.
Now that we’ve got that out of the way, let’s talk about if the child tax credit changes will be accounted for. And, unfortunately, it is probably a little too early to know exactly how this will be implemented. The IRS might direct employers to change the withholding automatically for employees that have declared dependents. Or they might revise the W-4 form quickly and ask employers to have their employees submit new forms. Or they might do none of the above, and either let you figure it out for yourself or wait until next April to receive the benefit.
If you like the hands-on approach (and it sounds like you do), then put the extra dependents on your W-4 and see what happens. Watch your withholding each paycheck and make sure it is what you want. If, in the future, your withholding drops suddenly without any action from you, then you’ll need to submit another W-4 to correct it again.
The government has chosen "none of the above." :) Instead, they will be sending out advance monthly payments from July - December 2021 totaling 50% of your expected 2021 child tax credit. You don't need to take any action to receive these advance payments, but if you expect your 2021 child tax credit to be smaller than would be indicated by your 2020 tax status (due to increased income in 2021 or your number of child dependents changing), you have the option of either updating your income information or refusing the advance payments altogether. More details are found at the IRS' Advance Child Tax Credit Payments in 2021 website.