It's not for the Child Tax Credit, that is a separate thing. You got 1,400 for each person on your 2020 tax return. 1,400 x 3 = 4,200. Then you each got a letter for half of it 4,200/2=2,100.
The Child Tax Credit will be covered separately. Did you get any of the Advance Child Tax Credit payments? You get a letter 6419 for that.
