My first thought was also Chinese, but I've looked at several websites with no mention of their being open.
Another thought would be a hotel, Speaker's Corner, not appearing to be open. Tara I didn't check as I thought it would be less casual for some reason.
How about Red Arrow? Since they are open 24hours?
I agree with dfrost. Have something you can either buy or make ahead and reheat that night, less stress on all. You and family will be out and about and just want to be home for the night, the in-laws driving to get there will too just want to be able to get there and relax. Even if you all enjoy sandwiches together so be it. You can always go out Wednesday if you all feel like it.
I thought there was a post last year "What is Open in Nashua Christmas" or something like that, but a search did not get any hits. Maybe it was another area.
I've been watching for a Nashua suggestion and sorry nobody has responded. Could be eating out on Christmas Day with tired children is not a good idea at all. Do you have a slow cooker? You could leave a pot roast or beef stew cooking. I think a chicken pot pie already cooked and just re-heated might also work. Have some vegetables prepped before you go out. Or maybe just have a good, hearty soup. Pick up some nice rolls or bread the day before. Also buy your favorite pie. Make some homemade cookies the weekend before. Just keep it simple. Good luck!