Yeah, if Mom and Dad are available to test, there's no reason to test the kids as well; the kids won't have any matches that Mom or Dad don't.

It is helpful for genealogical research to have multiple full siblings test when both parents can't be tested; my aunt and uncle have matches that my mom doesn't, and with their tests I can confirm an ancestor a few generations back that I wouldn't be able to confirm with my mom's test alone. But if my grandparents had lived into the DNA testing era and been tested, I wouldn't need the siblings' tests.