I have two kids, ages 6 and 4, and both sets of grandparents live one state away. Consequently we often drive there to visit.
The kids sometimes like to get a bottled drink when we stop at a rest stop, but I'd always worry they'd spill as we were driving. My solution: I keep flexible drinking straws in the car. With a straw in the bottle, the kids don't have to tilt the bottle back to drink, which reduces the liklihood of spills.
Even better, if they don't finish the drink in one go, they can bend the straw, push it into the bottle, and replace the cap! The bottle neck is usually long enough to keep the straw from being submerged.
Bonus hack: I keep the straws in a travel toothbrush holder in my glove box. They stay clean and straight!
After a particularly sticky juice spill, my in-car beverage policy changed to water only. And I finally bought some good-quality drink containers that fit in the car's cupholders.
Even so, I wish I would have known this hack years ago — because who remembers to bring drinks every time? Spilled water's no fun, either.
More: Hacks for car travel and road trips