No booster seats are FAA certified, because they don't work without a shoulder belt. There are harnessed seats that are (in fact almost all harnessed seats are, the only two I can think of that aren't are the SafeGuard GO and the Britax Regent, because they require top tether use.)
However, a 6.5 year old should be fine in a lap belt. I do suggest possibly getting a booster that either folds (like the Compass boosters) or comes apart to a back and backless (like the Graco TurboBooster), so that you can carry the booster on to the plane (or in the case of one that comes apart, pack the back in your suitcase, well padded with clothing, and carry on the bottom) so that you will not subject it to gate checking or unprotected/unpadded checking of any kind, and even if your suitcases get lost you have something for her to use at your destination. You should try out any booster for fit before leaving, of course.
You might also consider a smaller seat for your son, if you have the budget for it, just to have more room in your row. The Ride Safer Travel Vest, SafeGuard GO, Radian, or Safety 1st Avenue are good options depending on his weight and seated torso height. (Weight and torso height would help a lot in narrowing down recommendations!)
