Jeanette's option is a good one (and so cute!), and reminded me that I forgot about the "dust masks" part.

If you don't want to mail order something, if you can find an Asian "dollar store" near you, like a Daiso Store, they may have child sized "dust masks."

As many of you know, it is quite common for children and adults in some Asian countries, like Japan, to wear masks when they are ill (to keep from spreading their germs to healthy people). Also worn during spring allergy season. And in some countries, they are worn due to increasing levels of air pollution, like in Beijing and Shanghai.