As a kid I was taken from Yorkshire to Cornwall for holidays, not as many motorways then so a long journey, this is what my mum did for two liitle ones
We went to bed as normal, we were carried out of bed still in pyjamas into the car, I have no idea whether it was 10pm or 5am. In those days there were no seatbelts so we laid down on the backseat - obviously not a good idea.
We continued to sleep, or went back to sleep - I note you said yours would wake - if the car is warm, quiet and they are still in PJs then you might get them back to sleep.
When we woke up we would stop at a reststop / cafe / garage / motorway service area and have breakfast and get dressed.
We then played car games, yours might be a bit young, not sure.
Sometimes we would have an A4 envelope with a list of things, when we saw one of the things on the list, mum checked, and if we were right we could open one of the small gifts, individually wrapped inside the envelope.
The list had things like
a red car
a boat
a cat
you get the idea. I know yours are too young to read but maybe an, "if you spot a red car you can open a present" type thing would keep them busy.
The presents were things like a packet of sweets, a notepad and pencil, the kind of things you put in kids birthday goody bags.