MIL lives over 200 miles from us and it usually takes 6+ hours to get there when we visit once you factor in traffic, breaks for the DC (and us). We generally go there maybe once a year and the rest of the time she comes to us on the train (she doesn't drive) which is about a 3 hour trip.
She is turning 70 this summer and wants to have a big party, she's booked the date and wants us to go. Fine, except it's the Saturday before we go on our family holiday which was booked in December, long before we knew about the party. We leave for our holiday on the Tuesday after the party and are driving to a site in France over 2 days, it'll be a good 10 hour journey all in.
DC are 9 and 7 and not great car travellers, between car sickness, bickering, boredom etc it's not much fun but as an occasional thing we can all muddle through. However if we all go to MILs party we will end up sitting in the car for around 24 hours in 6 days which I think is too much. I'd be ok for us to take the train up and use public transport to get around as needed as the train is easier to manage (though it's quite pricey) but DH is insistent that he wants to drive and if I don't want to I can stay home 'to prepare for the holiday' 
So AIBU, would you do all that driving?