My ex-husband died on June 11,2009 and we have a 12 year old son together. My ex-father in law said that I can not attend the reading of the will.

I am not admitted in Missouri.  However, most states do not requires that a Will be formally read so I doubt that there is a legal right to be present if it is.  The Will will be presented to Court for probate - a distribution of the assets of the estate as indicated in the Will.  You can go down to the Probate Court in the county in which your ex lived before he died and obtain a copy of the Will for a small fee.   Sometimes the distribution of the estate to a minor  can be contingent upon who is the legal guardian.  Although you are the natural guardian of your son you should consider having yourself appointed legal guardian of his estate" as it is called in some ares, so that you can speed up distribution.  Also, if your ex was required to maintain a life insurance policy for your son's benefit upon his death then this money comes to directly from the insurance company and not through the estate if it was done correctly.  Insurance companies may require legal guardianship as well.  Seek legal counsel in dealing with all this.  It may be the buffer you need between you and your ex-in laws.     

If your ex-husband had substantial assets, you probably want to talk to a wills and probate attorney in your area.  You do have not only the right but the responsibility to protect your son's interests.  You need advice you can rely on, and that will have to be based on all of the facts of your situation.

I'm not a Missouri lawyer, and probate law and procedure does vary between states.  But in most states, a "reading of the will" is a tradition, not something required by law, and if that's so, then it's a family event and your ex-father-in-law can exclude you.  However, I am certain that the law does have in place what you'll need to find out what's going on and a way to be heard if your son's interests are being harmed.  Your attorney will know the landscape.