As far as I know, the reason for putting children in the back is seat due to the following reasons:
- The airbag is not properly sized and positioned for a child and can injure a child. An airbag deployment combined with a rear facing child seat can easily kill a child.
- In a frontal collision, the collapsing structure of the car can intrude in to passenger compartment, making the rear seats safer for anyone.
- The rear middle seat provides the best side impact crash protection. It's been called the orphan seat, since in a severe accident a rear facing child seat in the middle rear seat of the vehicle is often the only survivor.
With the airbag disabled, #1 no longer applies. #2 is somewhat obsolete advice, because modern vehicles have much stronger front end crash structures. Significant intrusion in to the front passenger compartment is not so common anymore. The stronger crash structure also exposes all passengers in the car, front or rear, to higher G forces in a crash. When a high speed front end crash does happen to the point where intrusion in to the front passenger compartment occurs, it's often at such a high speed that every forward facing passenger in the car, in the front or the rear, is killed. An older weaker car may have allowed the rear passengers to survive at the expense of crushing the front passenger compartment. Many compact hatchback cars have such small rear ends that they provide very poor protection in a rear end accident, especially if hit by a tall vehicle such as a SUV.
If the front seat does not have an active airbag, and it's a modern car, the only significant advantage of placing a child seat in the rear is if it is rear facing or placed in the middle seat. If the vehicle is a compact hatch back, it can actually be more dangerous to have passengers riding in the rear seats, and that includes children, especially in forward facing child seats.
There are currently no crash tests done for higher speed rear end crash protection.