When a child is too big for a child seat, secure the child in a rear seat using the lap/ shoulder seat belt. Have the child sit upright and all the way back, then answer the following questions.

Checklist
If you answer yes to all these questions, the child is ready to wear the lap/shoulder seat belt correctly. If you answer no to any question, the child needs to ride on a booster seat until the seat belt fits properly without a booster seat.
Booster Seats

If a lap/shoulder seat belt cannot be used properly, position the child in a booster seat in a rear seating position. For the child’s safety, check that the child meets the booster seat manufacturer’s recommendations.
Safety of Larger Children
Protecting Larger Children-Final ChecksHonda Civic Owners Manual. Side Curtain Airbags
The side curtain airbags help protect the heads of the driver and passengers
in the
outer seating positions during a moderate-to-severe side impact. The side
curtain
airbags equipped in this vehicle are also designed to help reduce the likelihood
of
partial and complete ejection of vehicle ...