The "brace" as you called it (many others call it a "heel") is on the back of the rear calipers -- on both wagons and sedans. This are meant to collide with a vertical*, triangular structure on the frame (really a unibody, but let's call it that anyway) just behind the heel. In a rear collision, the buckling frame pushes the triangular piece forward, and that impacts the heel, and the energy of the collision is redirected to the massive rear axle instead of further crushing the rear zone. Thus, this limits crumpling in the rear passenger area.
[ * It's vertical in order to engage the heel regardless of whether the wheel is high or low at the instant of impact.]

If you're driving a Volvo even partly because you trust their safety, then you have to trust Volvo engineers that they wouldn't offer an accessory that put your children in excessive danger -- Volvos (including the 240) have always been extensively crash tested. I've put my own children, and their friends, in such a seat countless times, trusting Volvo. I do so, much more than any other car manufacturer (except M-B, which I've also owned :-).