Well nothing really, but I wouldn't want to send them to a houseful of nayberhood wimmin.
They might not see ‘em as you would. ;-)
I wouldn't worry about that. Wimmin aren't wired that way. Wimmin are going to take one look at the cookies and think one of two things: 1) Cookies!! I love cookies!! or 2) Chocolate!! I love chocolate!! The nipple thing won't even cross their mind.
That's not to say their sons won't notice. But unless they're very young (in which case they'll think it's super funny) they won't say a word. They'll eat the boobie cookies first and quietly hold you as a HERO of mythical proportion for baking boobie cookies in the first place and slipping them into the cookie exchange. If strange youts start high-fiving you out of the blue... you'll know why.