There is no magic term or phrase that you must use to exclude your son or daughter (or both) from taking under your will, and different attorneys use different language to make it very clear that the testator wishes to exclude one or more specific people (not just children). For example, here is some language I have seen used in a MA will to exclude a specific child:
 
"No reference in this Will to my “children” or “issue” shall include XXXX, of New Boston, New Hampshire, or his issue."
 
Or, if you have had it with the whole pack of them, you can exclude all of your children by simply leaving them nothing and including this language:
 
"My failure to provide in this Will for any of my issue, whether born before or after my death, is intentional and not the result of any accident or mistake."
 
Some attorneys like to include language leaving one dollar (or some nominal amount) to the child in question, just to make it perfectly clear that the child was not inadvertently forgotten, but that is not necessary.  Nor is it necessary to explain why you are excluding the child from the will, and such language may just give the child something to argue about in court.  Hope all that helps.