My son was 9 months the first time we travelled to Florida on Virgin. He's 15 now and so things have probably changed a bit. We did get bulk head seats and had the crib thing that they provide for the large drop down table. He was a big baby so only just fit lying down but it was a good place for him to sit up in, played with his toys in it and it kept it all relatively contained! Now it was a quiet flight which helped and we had the cabin crew popping by to see if we were okay and interacting with him to keep him occupied too.
This will be harder with twins but try and minimize how much you have in your bag on the flight as juggling the babies, the bag, the paperwork etc., etc., etc., makes for a tired and cranky set of parents which ultimately affects the little ones. Easier said than done I know but the calmer you are the easier this will be.
Good luck, I'm sure you will be fine and you will all have some lovely family memories to cherish.