Just to add another point of view - I would do it. I mean, it's a bachelorette party, penis cookies / cakes / models are a very common thing for that kind of party. At least where I'm from. And it's just a penis. She didn't ask you for cookies of a child slave chained to a loom 
All the mothers you are doing cookies for for their kids birthdays have probably seen one in real life as well, so I don't really see why they should be offended by it.
On the other hand, things might be different here in Germany. People are usually not offended by something as normal as sexuality. If you really fear that it could have a bad influence on your business, and feel uncomftable with it, it might be best to follow your instincts.