Struggled with this for years and still do. It does get better as they get older and your learn more about their skin. A lot of it is genetics. Came to that conclusion once I had gone through a whole two years of blaming myself about not using the right nappies, creams, food, etc., anything and everything that was available at the time we tried. Later, had nephews who suffered through worse flare ups. So, it is something that runs in families, and environmental factors do make it better/worse.
All the above, but with the reminder that every kid is different, so it is trial and error, like so many other things with little ones. A cure,for one child may not work for another. Here are some additional things that helped us:
1. Kept the temp very low and dry though out the area where LO was. Moist/dampness and heat cause more sweating and make it more likely to get infected.
2. The steroid prescription creams work and so does the hydrocortisone as a preventative. Best to use them before it gets bad.
3. If still in nappies, we got rid of wipes and just used flannels and a spray bottle filled with witch hazel or tea tree oil diluted-I think burts bees makes it. Lots of health food stores carry it.
4. Bath every other day in tepid water and with goats milk soap and lotion afterward. I think the brand was Cana? This made a huge difference for us.
5. Cut out all dairy, including formula and didn't go back to cow's milk till several years later. For weaning we used goats milk too. Some kids have this triggered by wheat too, so our allergist recommended completely no solids for at least a year and then very gradually added in least allergic foods.
6. Organic cotton under shirts & tights (H&M) were great to keep a layer underneath that no matter what we put on allowed her skin to breath. Generally keep clothes very loose or no clothes at home.
washed everything in sensitive skin detergent, even our clothes and bedding.
7. Aveeno, aquaphor, loads of prescription emollients, we went through them all. The goats milk stuff was the only thing that really worked.
Hope that helps and I've been there and hate to see any parent or child suffer through it. It can seriously mess with you mentally as well, so please look out for yourself too and enlist all the help you can muster. It is really hard to see pics of other kids/babies when your kid is suffering from it and looking splotchy and exhausted from not getting a restful sleep. Caring for our kids is more intense and we need more support/understanding.