Even at the end, after all that talk of caring about each other and being beat friends, Rumiko doesn't even make a short appearance. She should've at least been in Kawai's place.
But saying unko wouldn't get a kid punished like that because it's not as bad a word in Japanese. Kuso would be closer in meaning to the expletive. Since they're both adults both poop and shit are fine translations. Only the context would effect which is used.
Wordhippo is a good thesaurus...