I got my 2nd stimulus it was only$ 600.00. I have a son who I support completely by myself( a dependant) I also pay child support for my 2 other kids. Why was I not given credit for my dependents?

the payment was based on ONLY the 2019 tax return.

However, there is a 'settle up' section on the 2020 tax return.  What is really occurring is the stimulus eligibility is based on the 2020 tax return income, filing status, and dependents under 17.  that result, less what you received from the 1st two stimulus payments is what you are due and will be a credit on your 2020 tax return. . if that result is negative, then you are due no more and do not have to pay any thing.  

on the two children you pay child support, will they be on your 2020 tax return? 

You will not get a stimulus for the children you only pay child support on and do not claim as a dependent. The other parent, who claims the dependency, gets the stimulus payment