hi, I'M Grams. Sorry about this. I don't know how or why kids get colds and such so frequently, especially healthy, well-nourished children like your grandchildren...but, yeah; it seems to be a part of childhood, doesn't it?
I personally do a form of "alternative health" known as Orthomolecular. A form of it was semi-popular for a while because of Linus Pauling, who was all about fairly massive doses of vitamin C for maintenance of good health and the prevention of disease.
The version I do pre-dates Linus Pauling. Its from the 50s, and it was started by a Canadian psychiatrist, Dr.Abram Hoffer. He put together vitamin cocktails to help people with Schizophrenia, often along with shock treatments and/or tranquilizers. These days, most more "severely mentally ill" people who try Orthomolecular do it alongside tranquilizers, which is what I've been doing. I'm down to --1-- psych drug daily, a newer ("atypical") tranquilizer at a moderate dose. Keep in mind; I was definitely state mental hospital material for a long time there, and now I'm lucid, stable, moving forward, etc., in part because of the supplements (I think, anyway).
Orthomolecular these days covers a lot of ground, from what I do (which was originally called "Mega-Vitamin Therapy for The Schizophrenias") to massive dose injections and infusions of vitamin C, as an add-on treatment for those undergoing chemotherapy for cancer.
For simply boosting immunity and (hopefully...) preventing colds, reducing the intensity of a cold, etc., high dose C seems to be fairly standard. Personally, as part of my "line up," I take 20 grams of vitamin C per day. I use the time-release version, so I take 10 grams in the morning and another 10 grams in the evening. By using the time-release, I seem to be able to get all the benefits from the vitamin C in only 2 doses, versus the 4-5 doses old school Orthomolecular experts recommended using either the normal release tablets or the crystals, powder, etc.
But...yeah...lots of vitamin C, on a regular basis, is supposed to be very beneficial. I think some Orthomolecular people also use zinc in their programs. I take a zinc and copper pill everyday (if one supplements zinc, its a good idea to take in some extra copper, as well...), with a meal. Selenium also seems to help the immune system, and selenium is included in a number of Orthomolecular protocols.
adding in vitamin E helps some people, too. Personally, I go for a high dose (1,000 IU) of the natural form vitamin E, with mixed tocopherols. That form of vitamin E is--at the molecular level--what one gets from diet, but at a much higher dosage. I take mine with a high fat meal, to ensure proper absorption.
other than that...I don't have many major suggestions. Sorry about that. For a while there, I added in alpha lipoic acid on top of my Orthomolecular protocol. Alpha lipoic acid is a potent antioxidant in its own right, and it also helps "recycle" other antioxidants. I always took mine with food, to prevent stomach irritation.
I'm not trying to push this on you, but I do think the Orthomolecular supplementation has helped me a good bit with all aspects of my health, physical and mental. here's a link for some free information, if you're interested:
www.doctoryourself.com
have a good one.