Sentence:  "And so," you continue, "I'm putting your diapers away because you can now wear <WSD>big boy pants</WSD> and do your poop and pee-pee in the potty chair all by yourself." 
Meaning of the word marked with <WSD> tags:  Pull-up underwear worn by a boy during and after toilet training, as contrasted to the diapers worn during infancy.