If my daughter is just 8 y.O., should she take the antibiotic for sinusitis?

May be. Doctors don't usually recommend taking antibiotics for sinusitis in an eight-year-old unless you try decongestants , nasal saline for approximate seven days but there are many different thoughts on this subject this is just my opinion.
Yes. If she really has bacterial sinusitis & not just congestion, antibiotics are the treatment of choice at 8 or 2 or 22.