Mandarin language classes for children will start again on Sept. 15 at the Dover Town Library and continue on Saturdays through the school year. The free classes, taught by instructors from Bilingual Boston, use songs, music, and games to convey their lessons, and are divided into two age levels. The class for children from 6 months to 4 years old is from 1 to 2 p.m., and the class for children 4 to 8 years old will be from 2 to 3 p.m. No preregistration is required.