The condition is Down syndrome – no apostrophe, no capital s.
See the person first, not the disability. We all have virtues and characteristics that identify ourselves. Your son or daughter will appreciate being described by them. Prefer the “person with Down syndrome” to “the Down syndrome person.” Always put the child before the condition.
Down syndrome is something the people “have.” Your child is not a “victim” of it, nor does he/she “suffer” from it, nor is he/she “afflicted” by it.
“Typical” is preferred to “normal” when referring to your child’s development.
Your child may have an intellectual or a cognitive disability. They are not “mentally retarded.”