There's a lot of theory that goes into this, but to give you a rough idea, in the first case ("the balloon is red"), kids learn that "red" is the name of a property, like wet, or sharp, while in the second case ("the red balloon"), kids learn that "red"  is more like a proper name, like "Tom" or "Heather." Think about it this way: knowing someone's name doesn't usually tell you that much - it's just a label that happens to get attached to them - but knowing whether someone is funny or boring, or whether a dish is mild or spicy, tells you a lot. Funny enough, whether kids learn "red" as something like a name or something like a property, depends entirely on how their attention is directed when they hear it.