The homeopathic oscillicocinum would be perfectly safe and easy to take for the whole family to take. It this the #1 flu remedy in Europe. You can take it at the first sign of flu symptoms and it will shorten the duration. You don't have to use but a few pellets. Put them in the cap and then put them under your tongue. Don't eat or drink for 15 minutes before or after you take it. Vitamin C can be taken in syrup form or chewable for the teen. You take that to bowel tolerance and because it is water soluble the body throws off any it doesn't need. Vitamin D is pretty essential and the suggestion to take vitamin k with it is valid. Just find a form for children. Children should all take a good multi prophylactically along with the C and D. Elderberry is a good anti-oxidant for kids for any illness. I am also partial to echinacea/goldenseal blend for colds and it is available in liquid or capsules so works for either kid. There are lots of good remedies available for children. Just read reviews. Colloidal silver is effective and easy to take for children but unless you make your own it is pricey. I am not a doctor...just a mom who raised 5 kids using alternative remedies. Learn by reading, reading, reading.