If your son is in a daycare in the same building with you I would suggest that you find out what their emergency evac plans are. I know that if I could get my child I would. So that being said how about some supplies for your Son? small toy, dust mask, water and even a juice box or two? Not to mention sanitary supplies. (diapers and such).
As the Father of 3 girls I am a little protective (my wife says over protective no such thing) I would want them under MY care and protection.