Haibara will, on noting Senku's clothing situation, at least fish out a pair of child-sized pants for him. "Here, you can have these too." She figures somebody like Senku won't mind the oversized polo, but a boy would rather wear pants than not, right?

Either way, it's being offered along with the book, which she readily hands over without a word as to his manners. She's used to being with children, it doesn't bother her.