We'll be travelling with our three year old daughter, six year old son, and our friend and her six year old son. We're renting a car, so we'll need to bring our car seats. Normally, both boys are in high-back boosters and our daughter is in a car seat with a five point harness.
Our plan for the trip is to take backless boosters for the boys, and check them, since they can't be used on the plane. Even though they're normally in high-back boosters, we feel that they'll both be okay in backless ones, which will be easier to take along.
For our daughter, we're bringing her car seat on the plane. A friend is lending us a little cart thing with wheels that we can strap the seat to, so we can just roll it through the airport. It'll be a lot easier to keep her in her seat if she's buckled into her car seat!