For colds, I rely on vitamin C and zinc. My daughter takes a vitamin supplement anyway that has added vitamin C and zinc, but in the winter months, we really load up on oranges, satsumas, other citrus fruits. Nuts have lots of zinc, so I also make sure she has plenty of raw nuts in her diet. You might choose not to offer nuts (I think the recommendation is actually no whole nuts before age 5? but personally I do with supervision), but nut butters are also good. Both vitamin C and zinc will reduce the length and severity of the common cold. I personally also take a vitamin C and zinc tablet for myself, plus during the winter, echinacea tablets or tincture. I do find it makes a big difference. The one time I wasn't taking it and got sick, I was sick for two weeks and got a double ear infection! Every other time, it was gone within 3 days and much less severe. I don't personally give my daughter echinacea though, and I'm not sure what the recommendation would be for that. I'd probably talk to a homeopath to get some advice first. I really just try to beef up her diet in the winter when I know she's likely to get sick. It's been a lot better this year (probably because it's her 3rd winter and she's been sick so much every other year that she has more immunity now), but I'd like to think it helps.