Understanding the Message

During weekly visits to my allergist, I’ve noticed a lot of inattentive parents with ill-behaved children in the waiting room. So I was impressed one day to see a mother with her little boy, helping him sound out the words on a sign. Finally he mastered it and his mother cheered, "That’s great! Now sit there. I’ll be back in 15 minutes." What did the sign say? "Children must not be left unattended."