The last time I washed LOs crib sheet, she had a residual "drool ring" when I pulled them out of the washer, you know, like the coffee ring on a counter, but on her sheets. They were also feeling stiff, even thought they were clean. I use a product called "Rockin' Green" on LOs dipes and it helps keep them from building up minerals and detergent in our hard water. I grabbed all of her sheets and soaked them for about 8 hours in this stuff then ran the wash and all the marks are gone. They are super soft. I think they had just built up minerals and detergents after 18 months of use. Anyway, if your LOs sheets are looking worse for wear, this might help refresh them. They sell samples also. I think RLR would work as well, but have never tried it.
This is something I learned about while taking care of LOs diapers and I wish I knew it before I tossed my super nice eqyptian cotton sheets that felt like silk because they smelled like my X-husband and no amount of washing helped. I bet this stuff would have gotten rid of it.