Petr pointed me in the direction of the Knowledge base article

I’ve also just had time to look at the hierarchical repeaters. The KB is based on the child elements having a reference id to the parent element which would mean in the form adding this template attribute to it. Is there no way of being able to see that it is a child node of the Event from the tree and add the relation that way?

I get this error when trying to bind to the repeater even trying to call a GetChildRows() method:

CS0117: 'object' does not contain a definition for 'Row'

Not quite sure why that is....
<asp:repeater id="rptEvent" runat="server">
<ItemTemplate>
<h2><%# DataBinder.Eval(Container, "DataItem.Title") %></h2>
<p><%# DataBinder.Eval(Container, "DataItem.Description") %></p>
<b>Event Cost(s)</b>
<table>
<TR>
<TD>Type</TD>
<TD>Cost</TD>
</TR>
<asp:Repeater id="rptEventsCosts" runat="server" datasource='<%# Container.DataItem.Row.GetChildRows("relEventsCosts") %>'>
<ItemTemplate>
<tr>
<td><%# DataBinder.Eval(Container.DataItem, "cost") %></td>
<td><%# DataBinder.Eval(Container.DataItem, "cost") %></td>
</tr>
</ItemTemplate>
</asp:Repeater>
</table>
</ItemTemplate>
</asp:repeater>

the .cs code is:

//create a new TreeProvider instance
TreeProvider tree = Functions.GetTreeProvider();
//read all documents of type "cms.menuitem" and "mobilestore.phone"
DataSet ds = tree.SelectNodes("/events/%",
TreePathTypeEnum.AliasPath, "cms.events;cms.eventscosts".Split(';')
, "");
//add parent-child relation among tables
ds.Relations.Add("relEventsCosts",
ds.Tables["cms.events"].Columns["eventid"],
ds.Tables["cms.eventscosts"].Columns["eventid"]);
//set data source for the parent repeater
rptEvent.DataSource = ds.Tables["cms.events"];
//bind data
rptEvent.DataBind();