This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development is uncertain. The modern form combines 子 (child) and 宗 (religion/sect), but the original meaning and component relationship are unclear.
A child 子 beside a religious sect 宗 suggests a family lineage or descendant within a clan, but the actual meaning is uncertain.
For ソウ, imagine a child 子 in a sect 宗 softly singing a soulful song: soul -> ソウ, though the meaning remains unclear.