Natural children are the biological children of their parents. This article discusses the inheritance rights of those children.
In Texas, in general, children inherit the estate of their parents if there is no valid will. The spouse of the parent will also inherit a portion of the estate. How much the spouse inherits depends on whether the spouse is also the parent of the children (more) or the step parent of the children (less.) Read here for a discussion of the kinds of property involved and what the spouse and children receive when there is no will.
If there is a valid will, the will determines who gets the estate. A testator does not have to leave his estate or any part of his estate to his children. He is free to leave it to anyone he pleases.
If there is no will, or the will is determined to be invalid by a court after a will contest, then the decedent is treated as if he died without a will. This assumes that there is no older, valid will that would be admitted to probate. To review the grounds for contesting a will to have it declared invalid, read here.