Honestly, since I found crayons melted on my car seats, I'm hesitant about letting the kids keep stuff in the car. Now, I make them take everything in each time we go home. However, if you are going on a road trip, it is important to take things that entertain the children.
The Kaboodle company makes auto entertainment organizers that attach to the back of the front seat. There are several styles available that you can choose from to contain the kids' treasures. You might also allow the kids to take a small backpack to hold toys, books and so forth. Make sure that there are no melting or flammable items in the things that are stored in the car.