[jaljayjar]

We left at 4am for a 13 hour drive just to Jacksonville...we stayed the night there and then drove down the beach another night before checking in the resort. I think it just depends on how you think she will do. My kids 3, 8, 10 all did very, VERY well. They slept in lounge/comfy clothes that are decent for public or stopping yet comfy to sleep in, so when we loaded up they just had to get in the car and fall back asleep. We used their cues/behaviors as to when to stop.
Also, in my situation DH was doing the driving first so he slept while I stayed up and packed and loaded the truck. I kept the kids up late with me so they would be tired the next morning which wasn't a problem at all because they are night owls. They slept until atleast 12pm and then watched movies and played games the other 5hrs. to jacksonville.
If she big enough for a backless booster seat? Those tend to give a bit more freedom since they aren't so confined by the shoulder area.
Good luck either way!