Checking the harness is tight, top of the child’s shoulder two fingers flat, give it a slight wiggle, know that harness is sufficiently tight. If with two fingers at top of shoulder can make it into a loop, the harness is too loose. If struggle to get two fingers between the shoulder and belt, it’s too tight. Ensure easy access for the child, as you get the child out make the harness much longer. It means the child’s arms come out easily and the seat is ready for the next journey. When children are wearing different clothing, coats and jumpers, by making the harness long, makes it easy to refit the harness and pull it tight.
The harness position is critical and they do vary between Group 0 and Group 1 seat. With a Group 0 seat the child’s shoulder can be slightly below the belt guide, up to an inch then up to the belt point and slightly over it. Until it is half way between to two harness points. At that point the harness would need to be rerouted and moved up to the higher position. The child can stay in the seat until the shoulders are half an inch above the top belt point. They will then be ready to move onto the next type of seat
With Group 1 forward facing seats, it is critical to move the harness when the child’s shoulder reaches level with the harness hole. Don’t allow the child’s shoulder to go beyond it and don’t move it up too high too early. Move it up one level at a time as the child’s shoulder reaches each harness hole.