"We pledge allegiance to the flag of the United States of America, and to the republic for which it stands, one nation, under God, indivisible, with liberty and justice for all."
No you don't. You're a child without the right or ability to pledge allegiance. Allegiance to a flag is a promise to enter into military conflict on it's behalf should that be neccesary. It is not as Barbara Dietrich pretends in http://www.jaredstory.com/pledge.html a promise "to be a good friend.". This might be it's meaning in a social context. We are however talking about a political context because the pledge is to a flag. In a political context allegiance means exactly that. It means that if the entity you pledged allegiance to is in a fight then you have to join in. No ifs, ands or buts. That's what it means, a promise to murder the State's enermies, regardless of wether said enemies are acting against you or even in a way you admire and/or benefit from. To require children to do this is both impractical and horrifyingly immoral. Does anyone believe that any of the 12 years olds that are forced to recite this pledge would make effective killers? Does anyone doubt they'd be dosed to the eyeballs with Ritalin if they showed any sign they would be?*
In addition to being stupid it's also rubbish as an oath. No judge would hold a child liable for a contract to buy a car, rent a house or procure the services of a prostitute, but the State wants them to swear to kill somebody. Why would anyone wish someone to swear an oath they did not expect to keep? At least not until they become adults.
When they mature this puts the swearer in a strange situation. If they've sworn as a child do they publically renounce their oath? If they do they face official suspicion and the condemnation of their neighbours. If they do not they are faced with the official and public presumption that they intend to keep the oath. But the oath was non-binding and obliges them to do things that may be against their conscience. On the other hand it is also (hopefully) against their conscience to go against their sworn oath. Either way they are forced to do something that they feel morally uncomfortable with.
Nor is this solely a problem with people who reject military service in general or a specific conflict in particular. By extracting the oath before their majority the State has removed from people the ability to make the committment when they are morally capable of doing so. Like someone forced to pay $100 to relieve the victims of Hurricane Katrina they no longer have the option to do so by their own free will.
Many people praise the oath on the grounds that it is a promise to do something that is noble and good. If that were so why extract such a promise before it can be legitimately made? If the cause is worth defending why not rely on the free will of the nation's citizens to defend it? If a cause does not to attract enough volunteers to defend it then might it be a bad cause to begin with? If it is a good cause and it does not attract enough volunteers to defend it is that not a sign the nation is doomed anyway? Either way a good cause doesn't need or cannot use the forced extraction of oaths, forced by violence or by preying on the vulnerability of children.
Even if the cause in which children swore was good it would still not be good to make them swear to do so. A child by definition doesn't have the capacity to swear binding oaths. They are not mature enough to realise the consequences of their actions. If someone told you that a six year old sold one of their kidneys for a years supply of ice-cream and chocolate you'd be horrified. You would think (rightly) that an adult took advantage of a child's ignorance and shortsightedness in an immoral fashion. So much more so for a child that sells his whole life potentially in this oath of "allegiance", for nothing more than a teacher's approval. Moral adults (by which I mean those whose morals have achieved maturity and also adults who are moral) don't trick children into making promises.
Now on to the practical aspects of taking the oath. A child of Ayn Rand (spiritiually of course) might ask "What's in it for me?". Only a fool gives their alliegance without something in return. But the State gives nothing to these children. Not even a promise they don't intend to keep. A medieval lord, arrogant in his power and ruthless in it's exercise, would not dare to make someone swear allegiance without offering protection in return. A vassal's oath was always accompanied by a lord's. Each promised to protect the other. But the State promises nothing. It's not "selfish" to insist on something from the State in response to the State getting you to swear to commit the most foul murders and take the greatest risks. It's entirely reasonable and neccesary if you are to look to your families interest. Most people regard looking to such interest as not only morally allowable but morally imperative. If you die in a foreign field that will be forever (fill in nation) without getting some benefit for your family to compensate for the risk and/or loss then you've let done your kin and should be ashamed.
Then there's the obscene assertion "one nation, indivisible". It is self-evident crap. A nation is made up of people associated, and what is formed by agreement can be disolved by agreement. This is obvious legally and morally because each member has the right of self-ownership. This right includes the right to leave associations subject only to the insistance of others that you keep contracts. If each self-owner withdraws his insistance the association has nothing left in it that can legally or morally compel you to stay. Of course the United States was not actually formed by agreement. So much the better for my argument, for to argue that those forced into an association can't leave it but those who chose it can is rediculous. Those forced into an association have all the rights to leave of willing participants, and more. If the "form of government" becomes a hinderance rather than a help to exercising your rights you have every right to abolish it. If you do not then the United States of America ought not to exist and pledging to it is both wrong and futile. If the "form" of the government is that it stretches over a large geographical area and that makes it harder to exercise your rights you are just as right to break it up into a number of smaller governments as you are to change anything else about it.
The pledge of alligance is not a morally uplifting and harmless exercise. It is a deceptively gained promise to defend regardless of the worth of that being defended. A promise to violate the rights of one's fellow citizens to seccession. A promise to do so in causes you have no idea you might have to support and which would horrify you if you did. It's wrong to do this to your children.
* Not that that would neccesarily hinder them in becoming such.