How can a person get old poop stains out of baby clothes?

Look for a carpet cleaner called Folex, I found it at home depot for about $6 . I started using it as a stain remover , since it gets everything out of carpets , ink, blood , rust & it says old stains. It is made with just soaps, no harsh chemicals, I know this because I had to call poison control, not because my grandkids drank it, no , my sister asked to try it & put it in a water bottle, then later in the day she reached into her bag to get a sip of water , and you guessed it , she grabbed the wrong bottle. Duh...she was fine , lol...

Give it a try

Zout it out or Oxyclean could be helpful.

I have used ERA detergent I find gets out a lot of old stains.

PEROXIDE in a spray bottle, applied the day it stains, otherwise spay prior to laundry. This is the ingredient in all those oxi-clean pr0ducts.

WHITE VINEGAR will remove grease. Or use a cup of white vinegar in laundry. To soak use 1/4 cup each in water with Era or Dawn