No one in the user forum can see your information so we do not know why you only got the CTC for one child. You have not mentioned the ages of the children. If you have a child who will turn 18 at any time in 2021 you will not receive the CTC for that child. Or if you have a child who was born in 2020----did you claim that child on a 2020 return and say the child lived with you for the whole year? Does your 2020 tax return show that both children got the child tax credit on your tax return?
Look at your 2020 Form 1040 to see the child-related credits you received
Child Tax Credit line 19
Credit for Other Dependents line 19
Earned Income Credit line 27
Additional Child Tax Credit line 28
Child and Dependent Care Credit line 31 (from line 13 of Schedule 3)
If you are eligible to get the CTC for both children and somehow are not getting the monthly amount for both of them then you will be able to get it on your 2021 tax return.
This is a brand new system thrown together by the IRS after Congress changed tax laws in the middle of the tax season. There are going to be bugs, glitches and mistakes made and folks will unfortunately have to be patient until they sort it out.
If you do not receive the monthly payments you will be able to get the CTC on your 2021 tax return.
They may have used your 2019 return to determine the amount of your CTC---you can try updating your information on the portal if they are not sending the CTC for the younger one.