Actual medicine (steroids) as needed of course, but when thinking of alternatives, remember that skin issues can be a symptom of what's going on inside the body as well as caused by things that touch the skin on the outside iykwim.
Dietary triggers are very individual, but dairy and foods made of grains seem to be very common triggers. It may be worth getting rast tests done, but antibodies to a food needn't mean excluding it forever, but a few months then trying it again would be sensible.
Our 4yo DS couldn't take cows milk until very recently, but was fine with goat (about 25% of people who react to cow will also react to goat, and there's a lot who react to soya too). He also reacts to Maize(corn).
A broad spectrum probiotic might be worth a try - you can get powdered ones for kids that sprinkle on to food. There's a lot of research coming out recently about how gut health and gut bacteria affect health.
If you can, try organic skin and bath products- they don't have the SLS, parabens etc that most "normal" ones contain. Dr.Organic have a really wide range of products and they are lovely - our local Holland and Barratt stocks the whole range. They are mid-high price but I find a little goes a long way!
2 of our DC have reacted to the creams/lotions recommended - it was the lanolin in them, even tho it's purified.
Washing, non bio, and NO FABIC CONDITIONER! And wash at higher temps. 40 or preferably 60 to break down allergens like pollen and mould spores, animal dander etc.