Are you sure it should be $2,400? What is your filing status, and how many children under 17 are you claiming as dependents on your 2020 tax return?
For the second stimulus payment, you only get a payment for a dependent child who was under 17 years old at the end of 2020. If you are claiming a child or other dependent who was 17 or older as of the end of 2020, you do not get a payment for that dependent.
