We use normal detergent for our toddler's clothes. Our toddler regularly develops skin rashes over most of his body; we use creams prescribed by an allergy specialist to keep the rashes under control.
I'm allergic to SLS (I get tons of canker sores when I use regular toothpaste), so I suspect my toddler may be allergic to such detergents.
What's the best way to test? If the answer is "wash his clothes with a natural or mild detergent", please note that natural SLS (e.g. Tom's of Maine) iritates me just as much as synthetic SLS, and liquid soaps with SLS are often labeled as mild/gentle. I am very interested in hearing of specific laundry products which are known to not have any detergents.