There are no FAA regulations about who needs to use a car seat. The FAA STRONGLY suggests that children under 40 lbs (regardless of age) be secured in a FAA approved car seat during flight. Basically, if a child has graduated to a belt positioning booster then they are fine without one during flight, since they aren't FAA approved car seats. Also, you need to ensure your car seats are FAA approved. Usually there is a sticker on the side that indicates this, and it's listed in your owners manual. Now, if you do bring a car seat on the plane, there are only certain places on the plane where they can be used. The exit rows and the rows directly preceeding and following them aren't allowed. All car seats need to be located in a window, as they could possibly impeed exit of other passengers (yes even the parent) during an emergency. Since you have such small children, you might have to split up, so have two windows, so you can get two car seats in. The 5 YO is probably fine, and if you want to bring your booster, check it with luggage.