Dependents are entered or edited/deleted in MY INFO.
Make sure you have entered your child as a dependent in My Info, and that you have entered the child's Social Security number. Careful— do not say that your child’s SSN is not valid for employment. If your child was born in 2025 make sure you said he lived with you the whole year. There is an oddly worded question that asks if the child paid over half their own support. Say NO to that question.
If your dependent was a full-time college student, make sure you were careful on the MY INFO screen for “Uncommon situations” and that you indicated there that they were a student.