Re: What kind of seat for older kids?
Do you trust the girls to sit correctly in a booster the whole ride every ride? Most 5-6yo kids are mature enough to sit in a booster.
I'd definitely recommend a high back booster so they have belt guides for the shoulder belt and the side impact protection.
Graco Turboboosters are great.
Other good fitting boosters tend to be:
Do you have a BRU or BuyBuy Baby near you? Take the girls to a store that lets you try the seats in the car. You want to see the seat belt flat down on the thighs and a shoulder belt across the collar bone rather than off the shoulder or up on the neck. Pick out what you and the girls like and fits in your budget