A basic belief for the Christian Right is that America is a 'Christian Nation.' This belief is one of the most important foundations of every other position they advocate. So long as people believe that America is a 'Christian' nation, it should be easier for them to get the government to adopt laws which are based upon or which promote specifically Christian beliefs or doctrines.
Schools are an important component of their efforts to encourage this belief. Christian schools teach the doctrine outright, but secular public schools cannot -- at least, not yet. For the time being they can only promote a very vague sort of theism which the courts turn a blind eye to by pretending that it's no longer truly religious.
The Pledge of Allegiance, with it's added phrase 'under God,' plays a significant role here because every day most children recite what amounts to a patriotic affirmation incorporating a religious oath. In this manner children are encouraged to see patriotism and theism as essentially linked. Since most tend to come from at least a general Christian background, the Christian god is the only sort of god they'll have in mind when they say or hear 'under God.'
Even if children do not actively and consciously believe that patriotism requires theism, that America is specially blessed by God, or that Christianity is America's defining religion, the attitudes and assumptions necessary for such beliefs will be inculcated over years of repetition. When they are needed, they will be there and they will be ready to exploit.
This image is based upon a World War II poster stating that 'Teaching is War-Work, Too.' I have replaced the original text with advocacy of the concept that America is a nation 'Under God' and that you can't be a real American unless you believe in God.