My 12 yr old has a dime size "lump" under her chin in the soft area before where her throat starts. She sd its tender. I'm scared, what could it be?

Lymph node or cyst. Chances are it's a small lymph node or harmless fluid filled sac called a cyst. Unless your 12yr old is also having fevers, it's getting to be very painful or it came on very fast, it's okay not to panic and have them see their doctor in 1-2 weeks. It might even be gone by then!
Several things. If in midline could be a cyst that is present from birth and has changed recently.This is called a thyroglossa duct cyst.It could be a swoolen gland from trauma or infection.It could be a nodule on the thyroid gland. See your doctor.