N.H.
Is this your first trip travelling together w/all the children? If so you may just sit them down before the trip & just say, "listen I realize this will be a long trip but we'd really appreciate it if you would help w/the baby when asked" & just explain, if they're not ones who like to help w/baby (some kids are not) just explain that you will be busy sometimes & can really use their help to make the trip go smoothly like keeping the baby pre-occupied. If they are kids who don't mind helping then that's great, just remind them that you may need their help pre-occupying their baby brother. When we were kids & had to travel, our mom bought us those magnetic games like 'magnetic checkers' that the pieces are magnets & stick to the 'game board' which is also magnitized. If they don't like that, perhaps listening to MP3 players or person music headsets/players, portable dvd players, just something to make the drive not seem as long. You can also play dvds that your baby might like too although he is young, they still like to be entertained. Hope this helps, good luck & have a great trip.