You will need to stop along the way, perhaps at a shaded rest area with picnic tables or at a park so that you can eat. It isn't wise to feed a child riding in a car seat. If the child starts to choke, you may not be able to get to her in time to administer first aid. Stopping for meals or snacks is also a good idea nutritionally because it will keep food from being used as entertainment, as it often is in travel. Giving children snacks to keep them quiet in the back seat is sending the wrong message as to the role of food.
Drinks are fine while riding.
Water varies around the country, so pack a jug of water from home or bring bottled water. Pack individual juice containers that don't need refrigeration. The juice can be transferred to a bottle or a sippy cup. Some good travel foods are those that mother nature has already packaged, like bananas, oranges and hard-boiled eggs. Try to stick with foods that are easy on the tummy, like bread, bananas, individual containers of applesauce, crackers, and yogurt.