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.