You can message your child’s teacher once he or she has sent you a message. Family members are not currently able to message teachers before the teacher has sent the first message.
In the Seesaw app, tap ‘Inbox’ in the bottom right corner, select a message, and then write your message in the box at the bottom of the screen. When you’re done with your message, tap ‘Send’.
Reference: Seesaw Help Center Families - How do I message my child's teacher?