Kubo does not use Graperank or similar graph-computed webs of trust. It uses an 'association' event to establish the guardian-subject relationship for 2 keypairs, and uses 'moderation events' which are lists for view-only and interact permissions containing people, places, and events. This system defines 2 concentric bubbles of what can be seen and interacted with online, in which your kid can navigate, make contacts, build a follow list, etc. If you trust someone else's judgement over their children enough (e.g. your brother), you can 'extend' trust to them, which adopts their moderation events into your configuration. These events need to be respected by either a client, bunker, or relay. Although the design has already improved since, here is the most recent and comprehensive article on Trust Extended Permissions:

Yakihonne

Operation Kidstr 03: Introducing TEPP - Trust Extended Permissions Protocol

This article introduces TEPP (Trust Extended Permissions Protocol), a proposed system for Nostr designed to create secure, "permissioned environmen...

Older articles on TEPP and Kubo: Operation Kidstr 02: North Star

Yakihonne

Operation Kidstr 02: North Star

This article proposes a foundation to guide safer internet tools for children within the Nostr ecosystem. By developing relays, clients, and bunker...

Operation Kidstr, 1: Circles and levels of trust

Yakihonne

Operation Kidstr, 1: Circles and levels of trust

This second article in the series outlines “Kidstr,” a framework for building a safe online environment for children using a three-level trust ...