It seems obvious, but a will only lists those children to whom property is being given in the will. A testator (the person signing the will) may have had other children to whom property had already been given. These children may not be named in the will. Sometimes they are named if only to state that they have already received their inheritance and are not intentionally being left out.
Do not assume every child is named in the will.
Do not assume there have to be other children either.