Pass the parent recipient's 12-character UEI (or legacy 9-digit DUNS).
Do NOT pass a recipient hash: the /recipient/children/ endpoint is the
one recipient endpoint keyed by UEI/DUNS, and it rejects hashes with
HTTP 400. Get the UEI from search_recipients() results (the 'uei' field
of the -P row) or from get_recipient_profile().

Returns the child recipients, each with its own -C suffixed recipient_id
hash usable with get_recipient_profile(). Useful for mapping corporate
structures (e.g. Lockheed Martin parent -> all its subsidiaries).

year: optional 'all', 'latest', or a fiscal year like 2026.

The upstream endpoint returns a JSON array; it is wrapped here as
{"results": [...], "total": N} to keep the dict-only response invariant.