Something could have happened to the proper-sized Wednesday pants BEFORE they left to go to the Whites', and JonBenet could have insisted, when Patsy was changing her, that she needed to wear the new pair, either because it was a special occasion and new would feel more festive, or for some childish (6 yr old) reason.
Even if she wore pants and a vest, maybe she was wearing tights instead of socks, so they knew the too-large underpants wouldn't fall down, and maybe they were pretty elastic. If she was running out of clean ones, seems to me it's the housekeeper's fault, although they probably didn't want her to know. The prior molestation (autopsy report) could have caused bladder irritation or infection and extra urinating, I'd guess. So she may have been crying over her ruined Wednesday panties, partly because she wasn't feeling well, and was unusually demanding and opinionated. Didn't she lie down for a while in Daphne's room? Was probably whiney. If the parents knew at the time she'd been molested, maybe they felt the need for a party, to get their minds off it, and surely it wasn't FW. But then, who knows?