I received advance childcare payments for 2. But I am only claiming 1 this year. Why does 1 child not cancel out the other?

You are required to enter the amount that you received for the advance payments.   You say you received advance payments for two children.  Now you are only claiming one child for 2021.   However, the CTC for 2021 is based on your 2021 income.   Is your 2021 income high enough to put you in the "phaseout" range or into the range that is high enough that you cannot get the CTC at all?

Q C2. Can my Child Tax Credit amount be reduced based on the amount of my 2021 income? (added June 14, 2021)

A2. Yes. The Child Tax Credit phases out in two different steps based on your modified adjusted gross income (AGI) in 2021.

The first phaseout can reduce the Child Tax Credit to $2,000 per child.

• That is, the first phaseout step can reduce only the $1,600 increase for qualifying children ages 5 and under, and the $1,000 increase for qualifying children ages 6 through 17, at the end of 2021.

The second phaseout can reduce the remaining Child Tax Credit below $2,000 per child.

For additional information on the amounts of modified AGI that reduce the 2021 Child Tax Credit, see Q C4 and Q C5, below.

Q C4. How does the first phaseout reduce the 2021 Child Tax Credit to $2,000 per child? (added June 14, 2021)

A4. The Child Tax Credit begins to be reduced to $2,000 per child if your modified AGI in 2021 exceeds:

• $150,000 if married and filing a joint return or if filing as a qualifying widow or widower;
• $112,500 if filing as head of household; or
• $75,000 if you are a single filer or are married and filing a separate return.

The first phaseout reduces the Child Tax Credit by $50 for each $1,000 (or fraction thereof) by which your modified AGI exceeds the income threshold described above that is applicable to you.

It depends on your exact situation. If your children are between the ages of 6 and 17, you should have received $3,000 in advanced child tax credit payments for both children. The 2021 return reconciles the entire child tax credit you should receive with the amount you already received. For one child, the entire credit is $3,000, but it's $6,000 for two. 

Since you are claiming only one, you have already received the entire child tax credit you were entitled to in the form of advanced payments, but if you claimed both you would be eligible for an additional $3,000 on the 2021 tax return. That $3,000 difference accounts for the entire difference between your mock return with both children and the actual return with only one child claimed.