Need to Create a JSON in the Specified Format

JSON Format --

{
  "records": [
    {
      "attributes": {
        "type": "Product2",
        "referenceId": "pref1"
      },
      "name": "Product 2",
      "SBQQ__Features__r": {
        "records": [
          {
            "attributes": {
              "type": "SBQQ__ProductFeature__c",
              "referenceId": "ref1"
            },
            "Name": "Feature1",
            "SBQQ__Number__c": 1,
            "SBQQ__MinOptionCount__c": 1
          }
        ]
      }
    },
    {
      "attributes": {
        "type": "Product2",
        "referenceId": "pref2"
      },
      "name": "Product 2",
      "SBQQ__Features__r": {
        "records": [
          {
            "attributes": {
              "type": "SBQQ__ProductFeature__c",
              "referenceId": "ref2"
            },
            "Name": "Feature1",
            "SBQQ__Number__c": 1,
            "SBQQ__MinOptionCount__c": 1
          }
        ]
      }
    }
  ]
}

Apex Class - 

public class JSonCLassForParent_Child {
    public List<Records_Z> records;
    public class SBQQ_Features_r {
        public List<Records> records;
    }
    public class Records_Z {
        public Attributes attributes;
        public String name;
        public SBQQ_Features_r SBQQ_Features_r;
    }
    public class Attributes {
        public String type;
        public String referenceId;
         public Attributes(String type,String referenceId){
            this.type = type;
            this.referenceId = referenceId;
        }
    }
    public class Records {
        public Attributes attributes;
        public String Name;
        public Decimal SBQQ_Number_c;
        public Decimal SBQQ_MinOptionCount_c;
    }
    public static String createJSONforParent_Child(List <Id> pIds){
        List<Product2> pList = [Select Id,Name from Product2 where Id = :pIds];
        System.debug(pList);
       List<Records> rec = new List<Records>();
       List<Records_Z> rz = new List<Records_Z>();
       Map<Id,Product2> aMap = new Map<Id,Product2>([Select Id,Name from Product2 where Id = :pIds]);
       System.debug('map is ' +aMap);
        List<SBQQ__ProductFeature__c> fList = [Select Id,Name,SBQQ__Number__c,SBQQ__MinOptionCount__c,SBQQ__ConfiguredSKU__c from SBQQ__ProductFeature__c where SBQQ__ConfiguredSKU__c In : aMap.keySet()];
        System.debug('feature is'+fList);
       // Map<Id,SBQQ__ProductFeature__c> productFeatureMap = new Map<Id,SBQQ__ProductFeature__c>();
        Integer count = 0;
        for(SBQQ__ProductFeature__c f : fList){
                        count++;
            if(aMap.containsKey(f.SBQQ__ConfiguredSKU__c)){
            Records r = new Records ();
            r.attributes = new Attributes('SBQQ_ProductFeature_c','ref'+count);
            r.Name = f.Name;
            r.SBQQ_Number_c = f.SBQQ__Number__c;
            r.SBQQ_MinOptionCount_c = f.SBQQ__MinOptionCount__c;
            rec.add(r);
           // productFeatureMap.put(f.SBQQ__ConfiguredSKU__c,f);
            }        
        }
        SBQQ_Features_r sbf = new SBQQ_Features_r();
        sbf.records = rec;
          Integer count2 = 0;
      for(Product2 a : pList){
            count2++;
            Records_Z r = new Records_Z();
             r.attributes = new Attributes('Product2','pref'+count2);
            r.name = a.Name;
            r.SBQQ_Features_r = sbf;
            //r.SBQQ_Features_r = productFeatureMap.get(a.Id);
            rz.add(r);
        }
        System.debug(rz);
         Map <String,Object> parentMap = new Map <String,Object> ();
         parentMap.put('records',rz);
         System.debug(JSON.serialize(parentMap));
        String jsonString = JSON.serialize(parentMap);
         String    finalJsonString = jsonString.replaceAll('_','__');
         System.debug(finalJsonString);
        return finalJsonString;
    }
}