LWC HTML:

<template>

    <div class=”slds-p-around_medium lgc-bg”>

        <div class=”slds-m-bottom_small”>

            <lightning-button
                label=”Collapse All”
                onclick={clickToCollapseAll}>
            </lightning-button>
            <lightning-button
                label=”Expand All”
                onclick={clickToExpandAll}>
            </lightning-button>
          
        </div>
      
        <lightning-tree-grid columns = {gridColumns}
                                data = {gridData}
                                key-field = “Id”
                                hide-checkbox-column = true
                                onrowaction={handleRowAction}>
        </lightning-tree-grid>

    </div>
  
</template>

LWC JS:

import { LightningElement, track, wire } from ‘lwc’;
import { NavigationMixin } from ‘lightning/navigation’;
import fetchAccounts from ‘@salesforce/apex/ExampleController.fetchAccounts’;

export default class Sample extends NavigationMixin( LightningElement ) {

    @track gridColumns = [{
        type: ‘text’,
        fieldName: ‘Name’,
        label: ‘Name’
    },
    {
        type: ‘text’,
        fieldName: ‘Industry’,
        label: ‘Industry’
    },
    {
        type: ‘text’,
        fieldName: ‘FirstName’,
        label: ‘FirstName’
    },
    {
        type: ‘text’,
        fieldName: ‘LastName’,
        label: ‘LastName’
    },
    {
        type: ‘button’,
        typeAttributes: {
            label: ‘View’
        }
    }];
    @track gridData;

    @wire(fetchAccounts)
    accountTreeData({ error, data }) {

        console.log( ‘Inside wire’ );
        if ( data ) {

            var tempData = JSON.parse( JSON.stringify( data ) );
            console.log( ‘Data is ‘ + tempData );
            /*var tempjson = JSON.parse( JSON.stringify( data ).split( ‘Contacts’ ).join( ‘_children’ ) );
            console.log( ‘Temp JSON is ‘ + tempjson );*/
            for ( var i = 0; i < tempData.length; i++ ) {

                tempData[ i ]._children = tempData[ i ][ ‘Contacts’ ];
                delete tempData[ i ].Contacts;

            }
            this.gridData = tempData;

        } else if ( error ) {
         
            if ( Array.isArray( error.body ) )
                console.log( ‘Error is ‘ + error.body.map( e => e.message ).join( ‘, ‘ ) );
            else if ( typeof error.body.message === ‘string’ )
                console.log( ‘Error is ‘ + error.body.message );

        }

    }

    clickToExpandAll( e ) {
        const grid =  this.template.querySelector( ‘lightning-tree-grid’ );
        grid.expandAll();
    }

    clickToCollapseAll( e ) {

        const grid =  this.template.querySelector( ‘lightning-tree-grid’ );
        grid.collapseAll();
     
    }

    handleRowAction( event ) {
       
        const row = event.detail.row;
        this[NavigationMixin.Navigate]({
            type: ‘standard__recordPage’,
            attributes: {
                recordId: row.Id,
                actionName: ‘view’
            }
        });

    }

}

Apex Class:

public with sharing class ExampleController {

    @AuraEnabled(cacheable=true)
    public static List < Account > fetchAccounts() {

        return [ SELECT Id, Name, Industry, ( SELECT Id, FirstName, LastName FROM Contacts ) FROM Account LIMIT 10 ];
      
    }
  
}

Output: