How to get child WSSID based on parent term ID using CSOM

Question

I have got parent term's WSSID by this method. How do i get its child terms ids like this to build my CAML query for search in a share point list using CSOM

private static int GetWssIdByTermId(ClientContext clientContext, string termGuidString)

    {
        int result = 0;
        Web web = clientContext.Site.RootWeb;
        List list = web.Lists.GetByTitle("TaxonomyHiddenList");
        var q = new CamlQuery()
        {
            ViewXml =
               String.Format(@" <View>
                        <Query>
                           <Where>
                              <Eq>
                                 <FieldRef Name='IdForTerm' />
                                 <Value Type='Text'>{0}</Value>
                              </Eq>
                           </Where>
                        </Query>
                     </View>",
                  termGuidString)
        };
        var items = list.GetItems(q);
        clientContext.Load(items);
        clientContext.ExecuteQuery();
        if (items.Count == 1)
        {
            result = int.Parse(items[0]["ID"].ToString());
        }
        return result;
    }