Thanks everyone for your replies, links, and advice! I wasn't planning on bringing the seats on the plane, I was going to check them with special baggage. We have multiple connections and the thought of taking them around connecting airports along with small kids and carry-on seems daunting. One of the legs is a redeye and I remember how uncomfortable it was to be right up next to a car seat on a short hop to LAX, mom needs some sleep too :)
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The vest sounds cool, I looked into and, but I am from Canada and they are not certified here :(
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I'm not comfortable having my kids in boosters yet, I want them to have a 5 point belt esp. when we don't know the roads or drivers very well, not to mention the other side of the road. And not keen on getting a seat from the car rental agency.