Hello,

In order to achieve this, you will need to use the CustomDataHandler class and in particular, its OnGetContent method. In general, the custom handler (CustomDataHandler, CustomTreeNodeHandler etc.) gives you ability to execute custom code when some CMS event occurs. The events of CustomDataHandler are applied to all data items that are stored to the database and the OnGetContent is a method used by smart search. It's called every time the document (or custom table, forum, user) is indexed.

The first parameter of the OnGetContent method usually contains a TreeNode object and the second parameter contains text that gets indexed by Lucene.NET search engine used in Kentico CMS smart search module (let's call it content). In this case, you will need get the Multiple choice field of the currently idexed document, parse it to get IDs of all fields, get appropriate records from Age Group based on IDs and add them to the content variable.

Let's assume that Age Group is a custom table and you use the following query to bind the Mupliple choice field:

SELECT ItemID, AgeGroup from customtable_AgeGroup

For this scenario, the OnGetContent method could look like this one:

public override string OnGetContent(object obj, string content)
{
     if (obj is TreeNode)
     {
        TreeNode node = obj as TreeNode;
         if (node != null)
         {
             string field = ValidationHelper.GetString(node.GetValue(<Multiple choice field)>, String.Empty);
              if ( !String.IsNullOrEmpty(field) )
              {
                  CustomTableItemProvider customTableProvider = new CustomTableItemProvider(CMSContext.CurrentUser);

                  string customTableClassName = "customtable.agegroup";                        

                  string [] ids = field.Split('|');
                  foreach (string id in ids)
                  {
                      CustomTableItem item = customTableProvider.GetItem(ValidationHelper.GetInteger(id, 0), customTableClassName);

                       content = content + " " + CMS.GlobalHelper.ValidationHelper.GetString(item.GetValue("AgeGroup"), "");
                  }
              }
        }
   }
   return content;
}

Now, you should be able to search for documents using values form the AgeGroup table (toddler etc.).

If you want more information about the CustomDataHandler class and how to add it to your project, please visit following links:
Event handling overview
Data handler

In terms of displaying correct values in transformation, you need to create a custom transformation method:
Adding custom functions to transformations

Another example of using OnGetContent which might help you understand what you need to do to make it work can be found in this knowledge base article: How to search for documents using assigned category

Best regards,
Michal Legen