Once children have outgrown a group 1 forward-facing seat, the best option is to use a group 2 or group 2/3 high-backed booster seat. Children using these seats are much less likely to be injured in a crash than children who are using seat belts on their own, or even worse, are completely unrestrained.
High-backed booster seats do not have an integral harness to hold the child in place. Instead, the car's seat belt goes around the child and the seat (except groups 1, 2 & 3 booster seats which have an integral harness which the child uses until they reach 18 kg in weight, when they should swap to using the car's seat belt with the booster seat).
The booster seat raises the child's seating position so that the car's seat belt fits them properly. They have red indicators or slots to show where the seat belt should go. There is usually one at shoulder height to position the diagonal part of the seat belt and ones at waist height, often under the booster seat's armrests, to help position the lap part of the seat belt.