If Henry dies with three living children, Abraham, Barbara, and Charlotte, then they are his heirs. However, if Abraham had died before Henry and if Abraham had children of his own (Yolanda and Zebulon), then Henry's daughters Barbara and Charlotte and Henry's grandchildren Yolanda and Zebulon would be Henry's heirs.
Barbara and Charlotte's children are not heirs of Henry at this point in time as Barbara and Charlotte are still living.
Let's say that Henry only had one sibling, George, who never married and never had children.
George dies the day after Henry. Barbara, Charlotte, Yolanda, and Zebulon are
Heirs do not have to be children. They could be nephews, nieces, cousins, parents, etc.
Of course, Henry or George could write a will leaving their property to someone else entirely. That person would be their beneficiary or legatee--not their heir. Heirs can be disinherited, but they are usually referred to as heirs or heirs-at-law.
Don't assume heirs are children or descendants. What heirs are depends upon the family structure.