I put appx a cup of oatmeal into a coffee grinder until it was of a smooth texture. Then I sprinkled the oatmeal into the bathwater. I also would take some of the ground oatmeal, mixed it with water to make a paste, then rubbed it on her skin. You can also purchase Aveeno oatmeal bath--- I just liked the idea of making it from scratch without any added ingredients or chemicals.
You may also look into getting a water filtration system for your house-- which is costly, but may help.
Best wishes...