Hi there, welcome to the Community! SWA seats are 17 inches between armrests, so your Britax seats might be a bit too big. Do they have an "FAA Approved" sticker somewhere on them? If your babies are big enough, I'd highly recommend looking into the CARES harness. It's my favorite way to strap my kids (age 1.5 and 4) into their seats on the plane. It's FAA approved, safe, and super convenient. http://kidsflysafe.com/ I mostly use this when I don't need to have my car seats with me (visiting family that have our extra seats, or using public transportation). Another thing you might consider if you'd rather a car seat is the COSCO Scenara (found at Walmart for under $50). It is REALLY lightweight (i.e. easy to lug around the airport) and definitely fits into the plane seat, forward and backward. When I had only my older child, that's what we used most of the time when flying. Don't forget to use Family Boarding, and feel free to send other questions our way! Happy travels! Sarah P.S. I did search and find that all Britax car seats are FAA approved. And, you can just raise the armrests. So your seats will fit. But I'd still have you to consider either of the options I gave to see if they might work for your family; I know it's hard work to carry kids, seats and carry on items all to the gate on your own. That said, I know you can make it work whichever you decide is right for you!
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