As flu activity is still picking up across the United States, there is a good chance that your kids have either had the flu, have the flu now, or will get the flu soon, especially if they didn't get a flu vaccine.
If your child does seem to have the flu, remember that he doesn't necessarily need a flu test if he has classic flu symptoms and there are a lot of flu cases in your area. Flu tests have a high rate of false negatives (a negative flu test when your child really has the flu), so your pediatrician likely won't rely on a flu test alone to diagnose your child with flu symptoms. It can be a clinical diagnosis - based on the your child's signs and symptoms.
And remember that although it can be expensive and kids often don't like the taste of the suspension form, Tamiflu can be helpful to treat flu if your child is diagnosed within 48 hours of the start of his flu symptoms. Your child will get better without an antiviral flu medication (Tamiflu or Relenza), but he may get better a little faster and his symptoms could be a little milder with one of these medicines.
Lastly, since flu season often goes into March or April, get your unvaccinated kids a flu vaccine to protect them for the rest of this year's flu season.