K.~
That's going to be a hard trip for your youngest. Their bodies can't hardly sit still for an hour and their minds don't know what to do with all that time. Your best bet would be to leave at night time so she will sleep the first half of the trip. If you start in the morning she will be over tired by night fall and not be able to fall asleep.
If you drive through the night she will wake up in the morning, you can stop for breakfast and lunch and dinner if you need to.If you stop at the golden arches you might find a play ground for both of the kids to run while you and hubby eat and they can eat in the car which will give them something to do and possibly put them back to sleep.
Also if you have a DVD in your car, try sing alongs or something interactive for the youngest when she is really bored and encourage her to play along with the movie.
Good luck and Happy Holidays
C. L
About me:
I am the mother of five my youngest is 14, also have been a foster parent and home stay for exchange students. I was a stay at home mom much of the time and homeschooled each child off and on to keep grades up (this worked well, I have and 18 yr old on her 3rd year in college!)
I've taken that trip with 3,4,5, and 6 kids and night time is best.