A child under 150 cm must not sit in a front passenger seat with an active airbag
A child under 135 cm must at all times sit in a car seat that is appropriate for it's height and weight. Safety belt alone is not sufficient.
Car seat classifications depend on the child's weight
A child's head must never reach above the seats's top edge
The safety belt must not be twisted and should sit comfortably against the child's body. A safety belt must never be placed behind the child's back or under it's arm.
To read more, visit The Icelandic Transport Authority website.