A Kids profile filters the catalog to G and PG only. Children's is a curated-only genre that creators cannot self-select into.
Leaving a Kids profile requires the PIN, and the lock is stored on the user record — not in the query string. That distinction is the whole feature: a URL-based flag is escaped by deleting ?kids=1 from the address bar.
The filter holds everywhere, including watch parties: a Kids profile cannot join a room showing an adult title.