There are tons of bikes to rent all around the city. Just make sure to follow the rules of the road and stay in the designated bike areas (blue lanes). If you don’t, you will be chastised by the locals, and riding bikes in pedestrian zones may get you a ticket from the police. For kids’ bikes, as well as cargo bikes, you’ll need to reserve with a bike rental company, such as Die Rad Station.
More info on biking on the Vienna city page.