I absolutely love seeing family all through Christmas. When I was a child we saw my dad's family on Christmas eve for them to be able to enjoy some of our excitement. And I loved it. I now take my DS around to my parents' house for a few hours on Christmas eve so they can enjoy his anticipation, even though we will be spending a lot of the next day with them too. It's wonderful for everyone to share in the joy of a small child on Christmas eve. If my family were an hour away, I'd either go early in the day to see them and leave by late afternoon.
Or else, I'd do the traditional night time stuff earlier in the day. Like leave out cookies and hang stockings before you leave the house, so it's all ready for Santa. Then make a big deal of packing toothbrush, PJs, favourite teddies and special Christmas blankets in the car. Go to the party, have lots of fun until they are obviously tired. Then take your children to a different room, have them change into their PJs, have something like a glass of warm milk each, brush their teeth, say all their goodbyes. Then into the car to snuggle up in their blankets with their teddies and talk about how exciting it will be to fall asleep in the car and wake up on Christmas morning. Maybe have some gentle choral type Christmas music lined up to play in the car home. If they are anything like my guy, they'll be asleep before you get home and you can carry them into their beds once you get home. It's a bit different to the usual way of doing things but it could still be really magical for you all.