Access child aggregations from parent

I have below search

POST /MY_ID/_search?typed_keys=true
{
  "aggs": {
    
    "terms_agg": {
      "terms": {
        "field": "[RefNumber]"
      },
      "aggs": {
        "max_agg_startdate": {
          "max": {
            "field": "StartDate"
          }
        },
        "max_agg_enddate": {
          "max": {
            "field": "Enddate"
          }
        }
      }
    }
  },
...
...

I want to accesss buckets returned from Max aggregations max_agg_startdate & max_agg_enddate in upper level. Requirement is get the average difference between max_agg_startdate and max_agg_enddate. Lets say this calculation performs just above the "terms_agg", how do I get access to max_agg_startdate & max_agg_enddate from that level?

Thanks