I have already submitted my DS-160 for a USA nonimmigrant visa; I am travelling with my family and children, I got the appointment date, but I did not fill out a separate DS-160 form. Is it compulsory to fill out separateDS-160 forms, or what do I have to do?
Each member of a family needs their own DS-160, although a single appointment can be used. You should have seen the option to create a family application when you were signing up, according to travel.state.gov :
Q. I am traveling with my family or as part of a group. Can I create a family or group application?
A. Yes. When you select “Email Confirmation” on your Confirmation Page, you will be directed to a “Thank You” page. On the “Thank You” page you will see an option to create a family or group application. When you select this option, certain information from your application, such as destination, will automatically be imported to and displayed on a new application. Please note that if you use this option you must create an individual application for each family member or for each individual within the group.
However, if you haven't done this originally, it is still possible to create DS-160s for the rest of the family and then schedule an appointment for everyone.
I can't find anything about adding family members to an existing appointment. Immihelp says it is not possible, and that you will instead need to cancel your existing appointment, add the family members (with their DS-160 details), and then reschedule. There is of course no guarantee that you will be able to get the same appointment date, so you should do all this as soon as possible so you can get an appointment in time for your travel dates.