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.
