Query for all items and subitems in a board

Try this:

query getItemsAndSubItems($boardIds: [ID!]!, $groupIds: [String!]!) {
  boards(ids: $boardIds) {
    groups(ids: $groupIds) {
      items_page {
        items {
          id
          name
          subitems {
            id
            name
          }
        }
      }
    }
  }
}

Add the variables as required:

{
  "boardIds": [1234],
  "groupIds": ["topics"]
}

In the API playground, it will look like this:

Try there first.