Sentence:  You might say something like, “Nancy, this morning before you woke up, Dolly went pee-pee in her <WSD>big-girl pants</WSD>. We don't go pee-pee in our <WSD>big-girl pants</WSD>, do we?” 
Meaning of the word marked with <WSD> tags:  Pull-up underwear worn by a girl during and after toilet training, as contrasted to the diapers worn during infancy.