I do nutritional support, more for more symptoms, less if kiddo seems normally sick. Vit C to bowel tolerance, maybe extra zinc (I need to give this all the time), I'd probably give vitamin A in higher-than-normal doses for a rash illness, but maybe because my kids have never had a rash illness. Lots of other illnesses, never a rash. I'd think a B complex, as a multivit if you have one, just because, at least for my kids, it seems like they use up a lot of extra nutrients when they're sick.
Focus on foods that are easy to digest, homemade soup with stock, Aviva Romm's garlic lemonade, soups/stews, avoid foods that could be problematic, probably dairy unless you're positive it's fine.