YOU SAID:

I pooped my cousins pants while they wore them

INTO JAPANESE

いとこのズボンを履きながらズボンをはいた

BACK INTO ENGLISH

I wore my cousin's pants while wearing them

INTO JAPANESE

いとこのズボンを履きながら履きました

BACK INTO ENGLISH

I wore it while wearing my cousin's pants

INTO JAPANESE

いとこのズボンをはいて着ました

BACK INTO ENGLISH

I wore my cousin's pants

INTO JAPANESE

いとこのズボンをはいていた

BACK INTO ENGLISH

I wore my cousin's pants