This is a great question. In a situation where a parent dies without a will or trust and that parent had no spouse and was not divorced, then the only people who would inherit would be that parent's "issue." The legal term "issue" can be confusing but for lay people what the term "issue" means is lineal descendants. So a child of a parent would be considered "issue."
A grandchild of any parent who had a predeceased child would also inherit. In other words, if a parent had a child who has children, but the parent's child dies before the parent, then the grandkid(s) of the predeceased child would inherit.
Under California Probate Code section 240, when a parent in the above situation dies, that parent's share would be split among any surviving children or if any children predeceased the parent but left kids of their own, then those grandkids would inherit.
In a scenario where there were 4 children of the parent and 3 of the children survived but 1 died, and the 1 that died left 2 children then the division of the estate would be as follows: 25% to each child and the children of the predeceased child would that child's 25% share equally or 12.5% each.