Should young babies and children wear masks when the smog is bad in the city?

No: It is not practical to try to keep a mask properly fitted on a baby or young child. If a child is sensitive to bad air, keep his room (or the whole house, if possible) air clean by not having dusty items (fabric furniture, carpets, etc.). On bad air days, stay indoors and run the hepa filter. Invest in a car with a hepa filter, for when driving is needed. When traveling, bring a hepa filter along.

No: I doubt that a mask would significantly effect the air that actually gets into the lungs. They are too leaky around the sides and they might not trap the particles that are causing the most irritation.