Unless your twins are very tall, I'd still consider bringing their booster seats. Most children are not large enough for an adult seatbelt until between 7 - 9 years old. Here are the guidelines for determining if your twins are large enough for a regular shoulder/lapbelt combination.
When can my child be in a regular seatbelt without a booster?
Children are not ready to be in a regular lap/shoulder seatbelt until (guidelines from car-safety.org):
They are tall enough so that their legs bend at the knees at the edge of the seat; and
They are mature enough to remain seated with their backs flat against the back of the seat and not slouch; and
The lap belt sits high on the thighs or low on the hips (NOT on their tummy!); and
The shoulder belt crosses the shoulder and chest (NOT on their arms or neck!); and
Each passenger must have their own lap and shoulder belt! Never allow children to share a seatbelt.
Some organizations will also give limits like 80 or 100 pounds, 4'9" in height or 8 years old. These are rough guidelines, not absolute limits. The criteria above are most important.
It may be easiest to bring backless booster for your twins. These are relatively small and inexpensive. They should slip into your luggage and quickly install in a rental car.
Good luck and have a wonderful trip,
Jennifer
