hiya, we just did a 15 hour car trip with a 4 month old and a 3 1/2 year old. It went really well, because we travelled overnight!
DS slept until 3 am and cried for a feed, so we had a break then. DD woke sometimes when we stopped to fuel up or rest, but we kept going and she back asleep again. DH and I just kept swapping driving.
good luck!