It sounds like you may have accidentally checked the box indicating it was not valid for employment- you can go back to My Info to correct this.
Navigate to My Info, Your household, select your dependent.
- Choose "My child", continue and be sure their name, date of birth and proper citizenship is complete.
- Continue, and answer the question about living arrangements and parents.
- Continue, and answer any less common situations, if applicable.
- Lastly, enter the child's SSN and be sure that "Not valid for employment is printed..." is NOT checked.