I put it that way to note that they do have children (or spouse, brother, relative, etc.) but we don't know their names. Unless you can think of a better way to note that? Unfortuantely trying to use "unknown" doesn't really work here as otherwise that could apply to everyone theortically that they have unknown parents (just never met them) rather "unnamed" notes they do recognize they knew their parent, sibling, etc. just we (the readers) were never told their names.
Eh, using "a" doesn't really fit well grammatically. For example, say you have someone who was married more than once but didn't know the names of one of them. It'd read like "a wife (deceased), Lily" which doesn't go well together. Essentially "a" only works if there's a single name in that related field.