@RestResource(urlMapping='/BulkInsertaccountcontact/*')
global class Bulkinsert_accountcontact
{
   global class RequestBody 
   {
       Account a; //create single account
       List<Contact> cList;
   }
   
   global class myWrapper
   {
      List<Account> a;
      List<Contact> cList;
      public string status;
      public string message;
   }
   
   @HttpPost
   global static myWrapper doPost(RequestBody rb)
   {
      RestRequest req=RestContext.request;
      RestResponse res=RestContext.response;
   
      myWrapper response=new myWrapper(); //this is object of wrapper class
      
      List<Account> myList=new List<Account>();
      
      try
      {
        myList.add(rb.a);
        
        response.a=myList;
        
        for(Contact con:rb.cList)
        {
           con.AccountID=rb.a.ID;
        }
        
        //we need to insert contact List also
        Insert rb.cList;
        response.cList=rb.cList;
        response.status='Success';
        response.message='Created Successfully';
        
      }
      catch(exception ex)
      {
         response.a=null;
         response.cList=null;
         response.status='ERROR';
         response.Message='could not create record'+ ex.getMessage();
      }
      
      return response;
   }
}



{ "rb": { "a": {
			"Name": "LG1",
			"SLAExpirationDate__c": "2016-4-21",
			"Active__c": "Yes",
			"SLA__c": "Platinum",
			"SLASerialNumber__c": "200"
		},
		"cList": [{
			"FirstName": "K1",
			"LastName": "K2"
		}, {
			"FirstName": "K3",
			"LastName": "K4"
		}]
	},
	{
			"Name": "LG2",
			"SLAExpirationDate__c": "2016-4-21",
			"Active__c": "Yes",
			"SLA__c": "Platinum",
			"SLASerialNumber__c": "200"
		},
		"cList": [{
			"FirstName": "K5",
			"LastName": "K6"
		},
                {
			"FirstName": "K7",
			"LastName": "K8"
		}]
	
}

Error:
message: Unexpected character ('{' (code 123)): was expecting double-quote to start field name at [line:16, column:3]
errorCode: JSON_PARSER_ERROR

        { 
            "Name": "LG2",
            "SLAExpirationDate__c": "2016-4-21",
            "Active__c": "Yes",
            "SLA__c": "Platinum",
            "SLASerialNumber__c": "200"
        }

"foobar" : { 
            "Name": "LG2",
            "SLAExpirationDate__c": "2016-4-21",
            "Active__c": "Yes",
            "SLA__c": "Platinum",
            "SLASerialNumber__c": "200"
        }