They really didnt attacked them 2 obtain Renesmee, they just got together all the Vulturis and placed a day 2 go 2 the Cullens house.
Of course they were thinking Renesmee was dangerous, that changed when Alice and Jasper showed them Nohuel
They didn't attack to obtain Renesmee. It was Edward and Alice that Aro wanted, or their powers, for his "collection" in the Volturi guard. The Volturi just used Renesmee as an excuse, because they had been mistakenly informed that she was a human child who'd been turned into a vampire. They would have fought the coven, executed the rest and Aro would claim that Alice and Edward's thoughts were repentant in order to spare them. Then another member of the guard (Chelsea, I think her name is) would use her ability to change Alice and Edward's loyalties, making them willing to join the guard.
