L.L.
We had this issue for a while recently not only with the five year old but my seven year old as well! frustrating to find the underwear again and again. We encouraged and sent them back more often than not as we saw them leaving the bathroom. Then nightly we had baths, showing them the dirty underwear and telling them that it woudl cause them to itch and get rashes. Every time we saw them "fix" their underwear during the day we reminded them if they whiped EVERYTIME that itchy wouldn't happen. Also at night when the underwear was dirty talked about germs and compared it to sticky dog pop we find when we go walking in the park. It took a few months, but we (as parents) suddenly realized it had worked for both children. Good luck