For child safety, move the front seat as far forward as possible to install a booster seat, then move it back as far as it will go, without allowing it to come into contact with the child.
Check that the booster seat is resting against the back of the vehicle seat.
Children, and adults, must be correctly seated and strapped in for all journeys. The children being carried in your vehicle are your responsibility.
A child is not a miniature adult. Children are at risk of specific injuries as their muscles and bones have not yet finished growing. The seat belt alone would not provide suitable protection. Use a booster seat and ensure you use it correctly.