So if the only way you get the additional child tax credit is when child tax credit can't be used what's the big deal about it ? There's no additional money right ?

The child tax credit is $2000 per child.    First the credit is put on line 19 and is used to reduce any tax you owe.   Whatever -- if anything-- is left of that $2000 after your tax liability is zero, can be used as the "additional child tax credit" and can be added to your refund.  That amount goes to line 28 of your Form 1040.  However, that amount is calculated based on how much you earned by working.   You have to take the amount of money you earned from working, subtract $2500 from that, and then multiply it by 15% to see how much your additional child tax credit will be.  It cannot be more than $1600, and might be less if your earned amount was low.