Wearing a face mask could be an option to introduce to your child. However, you would have to be a little innovative in your approach as your child is more likely to pull the mask off, at the first sign of discomfort.
Telling your child, its part of a spy game, where he/she is a secret agent is ok, if your child is still a toddler, but this probably wont work for older kids. With older kids, you have to make them understand the importance of wearing the mask to keep the flu germs away.