Side airbags, curtain or torso, can be dangerous/fatal if someone is sitting/leaning in their path. On the other hand, not having them could increase the chances of the crash impact being fatal, so it's a tradeoff. If a child is strapped in a child-seat, he/she should be fine, as it won't be possible for him/her to position him/herself in the path of the airbag. Older kids need to be made aware of the risks and behave appropriately. Although the rear passengers may have more steel around them that makes it less likely a side impact will squish their bodies directly, the airbag's purpose is to reduce body acceleration/deceleration rates, not prevent the bumper of a truck from impaling the passengers. Therefore, regardless of the car body's strength/rigidity, the rear curtain and torso airbags reduce the risk of injury.
The Prius does have a good safety score for its class. But, it is important to remember that although a 4-star rating may seem good, there is a very big difference between a 4- and a 5-star rating. 5 stars: 0-10% chance of a serious injury 4 stars: 11-20% chance of a serious injury So, the Prius scoring only 4 stars for frontal impact means it's possibly twice as likely to sustain a serious injury in one than in a car that scores 5 stars. So, if safety is your primary concern in vehicle selection, you may want to research vehicles with 5-star ratings. Here's a website with interesting info on this subject: Informed For Life
