Yes, you should include all of your children in your Form I-485, Application to Register Permanent Residence or Adjust Status. Additionally, each child has to file separate Form I-485 if he/she is immigrating with you (even if your child is a derivative applicant).
You must state the total number of all living children that you have (including adult sons and daughters) in Part 6 “Information About Your Children” of your Form I-485. You must indicate: current legal name of each child, his/her A-Number (if any), date of birth, country of birth, and whether this child is applying with you. Part 6 has space only for 3 children. So if you have more children, use the fields provided in Part 14 “Additional Information”.
According to USCIS children that are to be listed in Form I-485 include: all biological or legally adopted children, as well as current stepchildren, of any age, whether born in the USA or other countries, married or unmarried, living with you or elsewhere, missing children and those born to you outside the marriage.
Also, note that there is a discount if you are filing together with your child who is 13 years old or younger. In case you submit your I-485 applications in the same envelope the filing fee for the child will be reduced to $750, instead of usual $1,140.