A seatbelt was designed for an adult body. A child is not a small adult, and the belt does not know the difference.
Where the belt is supposed to sit
A lap belt is built to catch the strong bone of the pelvis, so the hips take the force of a crash. On a smaller body the belt rides up, off the pelvis and onto the soft abdomen, because the hips are not yet wide enough to hold it down.
Submarining
In a hard stop, a body whose belt sits too high can slide forward and down, under the belt, instead of being caught by it. The belt then loads the abdomen and the spine rather than the pelvis. Doctors see the result often enough to have named it: the seat-belt sign across the belly, and beneath it, injury to the bowel and the lumbar spine.
Why fit and position decide the injury
Whether a young passenger is hurt this way turns on things that look minor: how the belt fits, whether a booster raised them to meet it, where in the vehicle they were sitting. The same crash and the same belt produce very different injuries depending on the body wearing them. That is the ground crash reconstruction and biomechanics testimony are built on.
The lesson
When a young passenger's injuries do not match what the adults in the same car suffered, the difference is not a mystery. It is anatomy meeting a restraint built for someone bigger.
On belt fit, submarining, and the seat-belt sign in young occupants, see Abdominal Injuries in Belt-Positioning Booster Seats (PMC) and the literature on seat-belt syndrome in children. This is general information, not medical or legal advice.
