What I did was ask them all the time 'do you need to go to the loo?' because often they may not think they need to go until you ask them kwim? And as soon as I was within coo-ee of a toilet I would take them to see if they could at least try to do one. And definitely loads of spare clothes as well. Maz sells little mats that you can have in the car as well JIC they have an accident while you're driving.