No. Biden cannot raise the child tax credit. All he can do is talk about it. The child tax credit has to be increased by an act of Congress. Congress has not done anything -- and we do not know if or when they will.
I know it takes 60% in Congress to do that, but when all the smoke clears, and they do something in that vein to do just that, what is the re-do for those that already filed, that will naturally want to refile? Everything in this country right now is unprecedented, so I sure the heck hope turbotax can flex and react to this, if and when it happens. For you to sit there and say this is highly unlikely, strongly suggests you are out of touch with the volatility in this country.
Since you have already e-filed, your return is now in pending status until the IRS begins o accept or reject 2020 returns on February 12. We have no idea what Congress will do with the proposed increase in child tax credit or how they will implement it. We cannot speculate on what -- if anything-- taxpayers who have already filed will need to do if Congress changes tax laws in mid-season. All any of us can do is wait and see what they do.
Assuming Congress passes such a law and the President signs it, the IRS then has to implement procedures for claiming it and program it. After that, the IRS must approve TT's (and other tax prep cos) programming of the change. So nothing happens instantaneously.