KK is referring not to panties and briefs, but rather pajama bottoms.?? It should be rather obvious whether they are girl's panties or boy's briefs. ??
Here is the relevant passage from Kolar:
CSIs had written about finding a pair of pajama bottoms in JonBenéts bedroom that contained fecal material. They were too big for her and were thought to belong to Burke.
Foreign Faction, Who Really Kidnapped JonBenet, James Kolar, page 370