$2000 per child is the maximum.
The "additional child tax credit" is a different way of calculating the same child tax credit. The main credit can only offset taxes you owe, it can't be refunded if you owe less than $4000 of tax (in your case). The "additional child tax credit" can be refunded even if you owe no tax, but only if you have income from working. Since you already qualify for the full $2000 child tax credit, the "additional child tax credit" is ignored.
