Your child’s safety is your responsibility. You need to be firm about wearing a restraint even if your child doesn't like it.
Make sure your child’s restraint is properly fastened and adjusted before you start driving.
When travelling in a motor vehicle, make sure you and everyone else is always wearing a seatbelt. Your child will then see this as normal and also expect to be secured in their restraint.
On long trips plan plenty of stops so children can stretch their legs and take a break from their restraint.
If you continue to experience difficulty seek advice from an allied health professional. They will work with you to identify ways to solve the problem.
Mobility and Accessibility for Children in Australia Ltd (MACA) have information and resources available to parents and allied health professionals to support child and family transport needs: maca | Victoria (macahub.org)