One thing that has helped us tremendously is a daily antihistamine (she takes prescription Zyrtec). My daughter started exhibiting food sensitivities and eczema as soon as we started introducing her to food at age 7 or 8 months. She grew out of most of them by age 5 or 6, but she still can't have any cocoa products, even creams, or cantaloupe. She's 11 now.
After trying every "natural" product on the market, we now use 2 different types of (prescription) cortisone creams for her skin (one is gentler and for her face). She does fine with petroleum products (vaseline was our go-to before she started the cortisone), but reacts to any kind of essential oil. We also have to switch back and forth between Aveeno and Eucerin lotions, as she tends to develop a sensitivity to one after we use it for a while. Oatmeal baths help a lot in the winter, I just wrap about a cup of oatmeal in a washcloth and rubber band it shut and put it inthe water. She also uses the cloth to apply the glutinous-like stuff that "oozes" out of the cloth.
And every year she gets a mysterious rash on her face, and this year it made her eyes swell, so she is using steroid drops in her eyes for a while. Because her reactions range from red skin to her eyes swelling shut, we also always keep an epi-pen at hand.
This fall we started Vitamin D (5000 iu) and a B-complex in the morning with her antihistamine and I have seen a marked improvement in her skin overall. Multi vitamins didn't do any for her, but this combo seems to work for her.
It's trial and error, mostly.