LWC
<template>
    <div class="slds-box slds-theme--default">
        <div class="slds-text-color_inverse slds-text-heading_large" style="padding:0.5rem;background:#16325c">        
            Accounts
        </div>
        <table class="slds-table slds-table_cell-buffer slds-table_bordered slds-table_striped">
            <thead>
                <tr class="slds-line-height_reset">
                    <th>
                        Account Name
                    </th>
                    <th>
                        Account Number
                    </th>
                    <th>
                        Industry
                    </th>
                    <th>
                        Rating
                    </th>
                    <th>
                        Type
                    </th>
                </tr>
            </thead>
            <tbody>
                <template iterator:it={records}>
                    <tr class="slds-hint-parent" key={it.value.objAccount.Id} style="cursor: pointer;">                        
                        <td data-label="Account Name" data-record-id={it.index} onclick={hideAndShow}>       
                                            
                                {it.value.objAccount.Name}
                            
                        </td>
                        <td data-label="Account Number">
                                             
                                {it.value.objAccount.AccountNumber}
                            
                        </td>
                        <td data-label="Industry">
                                             
                                {it.value.objAccount.Industry}
                            
                        </td>
                        <td data-label="Rating">
                                             
                                {it.value.objAccount.Rating}
                            
                        </td>
                        <td data-label="Type">
                                        
                                {it.value.objAccount.Type}
                            
                        </td>    
                    </tr>
                    <template if:false={it.value.hideBool} key={it.value.objAccount.Id} style="padding: 5px;">
                        <tr key={it.value.objAccount.Id}>
                            <td colspan="5">
                                <table class="slds-table slds-table_cell-buffer slds-table_bordered slds-table_striped">
                                    <thead>
                                        <tr class="slds-line-height_reset">
                                            <th>
                                                Contact Name
                                            </th>
                                            <th>
                                                Email
                                            </th>
                                        
                                        </tr>
                                    </thead>
                                    <tbody>
                                <template if:true={it.value.contactBool}>    
                                   
                                        <template iterator:it={it.value.objAccount.Contacts}>
                                            <tr style="padding:1px;" key={it.value.Id}>
                                               <td>{it.value.FirstName} {it.value.LastName}</td>
                                               
                                                <td>{it.value.Email}</td>
                                                
                                            </tr>
                                        </template>
                                </template>
                                </tbody>
                            </table>
                                <template if:false={it.value.contactBool}>    
                                    <b key={it.value.objAccount.Id}>No Contacts found for this Account!!!</b>
                                </template>
                            </td>
                        </tr>
                    </template>                    
                </template>
            </tbody>
        </table>
    </div>
</template>




JS
import { LightningElement,wire, } from 'lwc';
import fetchAccounts from '@salesforce/apex/AccountController55.fetchAccounts';
export default class ExpandableTable extends LightningElement {
    
    records;
    error;
    @wire( fetchAccounts )  
    wiredAccount( { error, data } ) {
        if (data) {
            console.log( 'Fetched Data ' + JSON.stringify( data ) );
            this.records = data;
        } else if ( error ) {
            this.error = error;
            this.records = undefined;
        }
    }  
    hideAndShow( event ) {
        let indx = event.target.dataset.recordId;
        console.log( 'Index is ' + indx );
        if ( this.records ) {
            let recs =  JSON.parse( JSON.stringify( this.records ) );
            let currVal = recs[ indx ].hideBool;
            console.log( 'Current Val ' + currVal );
            recs[ indx ].hideBool = !currVal;
            this.records = recs;
            console.log( 'After Change ' + this.records[ indx ].hideBool );
        }
    }
}



Apex
public with sharing class AccountController55 {
 
    @AuraEnabled( cacheable = true )
    public static List< AccountWrapper > fetchAccounts() {
     
        List< AccountWrapper > listWrap = new List< AccountWrapper >();
        
        for ( Account objAcc : [ SELECT Id, Name, Industry, AccountNumber, Rating, Type,  
                                        ( SELECT Id, FirstName, LastName, Email FROM Contacts )
                                   FROM Account
                                  LIMIT 10 ] ) {
            
            AccountWrapper objWrap = new AccountWrapper( true, objAcc, objAcc.Contacts.size() > 0 ? true : false );
            listWrap.add( objWrap );

        }

        return listWrap;
         
    }

    public class AccountWrapper {

        @AuraEnabled
        public Boolean hideBool;
        @AuraEnabled
        public Boolean contactBool;
        @AuraEnabled
        public Account objAccount;

        public AccountWrapper( Boolean hideBool, Account objAccount, Boolean contactBool ) {

            this.hideBool = hideBool;
            this.objAccount = objAccount;
            this.contactBool = contactBool;

        }

    }
     
}