If you are looking for healthy drinks with minimum of fuss, I would recommend reselling juices like Odwala or some other fresh bottled juice. There used to be a brand called Fresh Samantha in the northeast, but they got bought out. That would give you healthy and no mess - just a fridge.
Also, juice is a good fit because kids generally are not into coffee because they have less refined palates. To make coffee taste good to kids, you'd need a lot of milk and sugar. That tends to work against your "healthy" requirement. Granted there is a lot of sugar in most juices, but you can probably rationalize that with all the vitamins and other stuff.
Also if you have a drive in, you'll be able to see if you customers have kids in the car. Ask them what they would like. Think of 3 to 5 options and ask them. Fresh juice, bottled juice, hot capp (blech - but kids like it), decaf iced coffee (you can make some flavored iced decaf coffee drinks that kids love) or smoothies. If you have a drivethrough, I would not suggest smoothies - it depends on how much room you have to work with and your ability to manage your inventory of fruits etc without spoilage costs that get too high.
Good luck. Let us know what you end up doing and how it goes.
-James
