[dragonwriter]

> It’s common for car seats to be attached to a seat belt which adds some play.

Proper installation with a seatbelt involves engaging the retractor so it has basically no play at all.

It's true that improper installation is quite common, though.

> The LATCH system is seems to be bolted on, but not always used. It’s still going to bounce in a crash as the seat cushion has a lot of give under these forces.

A forward-facing seat using LATCH should usually be installed with three point LATCH (the third is over the seat back), so without much play in any direction, certainly not forward.

Two point LATCH can move up-down and side-to-side a bit, but still not forward.

[Retric]

The 2 point LATCH system is attached below not behind the car seat’s center of gravity, so under heavy acceleration it depresses the car’s seat cushion and rotates forward then bounces up and back. Minimizing this is the reasoning behind the 3rd attachment point.

[dresstotheleft]

In forward facing child seats there are the two latch buckles as well as a tether anchor that runs over the top of the seat.