Wagtail menu with children

Unfortunately there's no examples of admin menu with submenu in Wagtail 6 docs, but here's Nigel van Keulen's solution from Wagtail's Slack #support channel:

# wagtail_hooks.py

from wagtail import hooks
from wagtail.admin.menu import Menu, SubmenuMenuItem, MenuItem, SubMenuItemComponent
from wagtail.images.wagtail_hooks import register_images_menu_item
from wagtail.documents.wagtail_hooks import register_documents_menu_item
from wagtail.admin.wagtail_hooks import register_collections_menu_item

# change app to your main app folder with settings/base.py
from app.settings.base import HIDDEN_SETTINGS_MENU_ITEMS, HIDDEN_MAIN_MENU_ITEMS


assets_menu = Menu(
    register_hook_name="register_assets_menu",
    construct_hook_name="construct_assets_menu",
)

@hooks.register("register_assets_menu")
def register_assets_menu():
    return register_images_menu_item()

@hooks.register("register_assets_menu")
def register_assets_menu_item():
    return register_documents_menu_item()

@hooks.register("register_assets_menu")
def register_assets_menu_item():
    return register_collections_menu_item()

@hooks.register("register_admin_menu_item")
def register_assets_menu_item():
    return SubmenuMenuItem(
        _("Assets"),
        name="assets",
        icon_name="folder-open-1",
        menu=assets_menu,
        order=300,
    )

@hooks.register("construct_settings_menu")
def hide_settings_items(request, menu_items):
    menu_items[:] = filter(lambda item: item.name not in HIDDEN_SETTINGS_MENU_ITEMS, menu_items)

@hooks.register("construct_main_menu")
def hide_main_items(request, menu_items):
    menu_items[:] = filter(lambda item: item.name not in HIDDEN_MAIN_MENU_ITEMS, menu_items)

# settings/base.py, adjust to needs

HIDDEN_SETTINGS_MENU_ITEMS = [
    "collections",
    "workflow-tasks",
    "workflows",
    "redirects",
    "menu",
    "groups",
    "users",
]
HIDDEN_MAIN_MENU_ITEMS = [
    "reports",
    "images",
    "documents",
    "wagtaildocs",
    "help",
    "snippets",
]