The law in short
Children under 135 cm must ride in an approved child seat or booster. From 135 cm they can use a regular seat belt. The driver is responsible for compliance.
The rules, the scientific recommendation, and practical installation.

Children under 135 cm must ride in an approved child seat or booster. From 135 cm they can use a regular seat belt. The driver is responsible for compliance.
Trafikverket's recommendation: rear-facing seat up to AT LEAST age 4. Rear-facing dramatically reduces the risk of neck and head injuries in a frontal crash, a child's neck can't handle the forces a forward-facing seat exposes it to.
ISOFIX means locks the seat directly into anchor points beneath the car's seat. Faster, safer, harder to install incorrectly. Belt install also works but requires careful reading of the manual. Look for the ISOFIX symbol in the seat crevice.
• Child wearing a thick jacket under the belt, in a crash the padding compresses and the belt is suddenly loose. • Seat not tightened enough, the test is it shouldn't shift more than 2.5 cm in the car seat. • Five-point harness too loose, no more slack than a finger can fit under. • Rear-facing seat tilted too steep, child's head can loll forward.