You can disinherit someone in your will. Typically, this is only required if you want to disinherit someone who would possibly be entitled by law to a part of your estate, such as a child, parent or other relative.
If you do give more to one child than to another, then you should state your reasons, in order to show that you thought out your plan. Otherwise, the one who received less might argue that you did not realize what you were doing and were not competent to make a will.
Your living parents are entitled to half of your estate if you have no children, as mentioned earlier. You cannot disinherit them without good legal reason, either. Examples of good legal reason include if they accused you of a crime or failed to furnish your bail when they could have done so. Consult a notary if you wish to disinherit your parents.