They will probably sleep most the way. I would feed right before you leave. then stop half way and take them out of the car seat so they can stretch and possibly eat again.
I went on an 8 hr trip when she was 5 months. If they're set in a sleep schedule, I'd try to wake them to leave early in the morning, so it doesn't mess them too much.
My husband and i took scarlett on a 16 hour road trip when she was 5 months old! She did great! Slept most of the ride (at night) then we took a 4 hour trip to wyoming a few weeks ago, she did fine!
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