Hi 

Please use this code

<apex:page Controller="accountClass">

    <apex:pageBlock title="My Content">

        <apex:pageBlockTable value="{!accountsToPopulate}" var="item">
            
			<!--For account Name-->
            <apex:column value="{!item.accRec.Name}"/>
			<!--For account Total Amount-->
			<apex:column value="{!item.Top_Level_Account}"/>

        </apex:pageBlockTable>

    </apex:pageBlock>

</apex:page>

------------------------------------------

Public class accountClass {
    public List<Account> accounts {get; set;}
	public List<wrapclass> accountsToPopulate {get; set;}
	
	public accountClass() {
	    accounts = new List<Account>();
		accountsToPopulate = new List<wrapclass>();
		accounts = [SELECT Id, Name, (SELECT Id, Amount__c FROM child_Accounts__r)
		    FROM Account];
			
		for (Account acc : accounts) {
		    Integer total = 0;
			If (acc.child_Accounts__r != null) {
			    for (child_Accounts__c childRec : acc.child_Accounts__r) {
				    total = total + childRec.Amount__c;
				}
			}
			accountsToPopulate.add(new wrapclass(acc, total));
		}
	}
	
	//wrapper class
	public class wrapclass {
	    public Account accRec {get; set;}
		public Integer Top_Level_Account {get; set;}
		
		public wrapclass(Account accountRec, Integer totalVal) {
		    accRec = accountRec;
			Top_Level_Account  = totalVal;
		}
	}
}