I would put them in a small pet carrier or a box with some sort of liner. If you do the box, make sure it has holes in it. Then get some damp substrate to put inside the pet carrier.
Make sure to not feed your turtles about 3 - 4 days before the trip to ensure that they dont poop on the way. A poopy mess is not something you want to add to it....
You may want to take a water dish with you just in case you have to spend the night somewhere or something happens. Also, take some pellets or whatever food it will eat that wont spoil (you never know if your car will break down). If you dont need them then great but if you do, then at least the turtle will be taken care of.
If you stop somewhere to eat, make sure that you leave enough ventilation in the car so it doesnt heat up.
Thats all I can think of....