Display child records when selecting parent record from dropdown list?

 By using Action Support function we can achieve this scenario

public class ActionSupportController

{

    public list lstoption {get; set;}

    public list lstaccount {get; set;}

    Public List lstContacts {get; set;}

    Public string selectedAccount {get;set;}

        public ActionSupportController()

            {

                lstoption=new list();

                lstaccount= [select id,name from account order by name];

                lstoption.add(new selectoption('','--None--'));

                for(account acc: lstaccount)

                    {

                        lstoption.add(new selectoption(acc.id,acc.Name));

                    }

            }

    Public PageReference LoadContacts()

    {

        lstContacts = [select id, firstname, lastname,phone,LeadSource,Department,Status__c,Email from contact where accountid =: selectedAccount];

        return null;

    }

}