= Solution
An individual can have any number of children, so use a <rose tree>, with one <list> of child <trees> at each root:
``
data DescendantTree = Desc Person [DescendantTree]
deriving Show
descendants :: Database -> Person -> DescendantTree
descendants db p = Desc p (map (descendants db) (children db p))
``
The root records `p`, its immediate child <trees> record `children db p`, and <recursion> repeats the same construction at each generation. An individual with no recorded children produces `Desc p []`. For a finite <database> whose parent-child <directed graph> is <acyclic>, every recursive descent advances along a path that cannot revisit a vertex, so the construction terminates. On inconsistent cyclic records it can unfold indefinitely; finiteness of the <database> alone is insufficient for termination.
Two branches may contain the same descendant, as happens when relatives have a child. This is appropriate for the requested <tree> unfolding; it preserves both lines of descent rather than silently choosing one. The concise constructor equation is
$$
\boxed{D(p)=\mathrm{Desc}\ p\ [D(c)\mid c\in\operatorname{children}(db,p)].}
$$
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