I want to capture the open event when a user clicks on the "+" button for a child layout in a grid's field layout

I have a xamdatagrid with multiple layouts simulating a hierarchical layout. I want to implement lazy load on data that contribute to the child layout, since it will be an expensive operation to generate that data. Inorder to accomplish this I want to know when the user clicks on the "+" button to open the child layout. To start with i will bind some dummy data that says "Loading Please wait.." so that the "+" button appears initially. Once i receive the event when the user clicks on the "+" button i can then go ahead and construct the data set that will bind to the child layout.  

I am attaching my xaml and also an image of the view as it appears now with some overlay showing what i am trying to accomplish.

<igDP:XamDataGrid
Name="ApprovalsGrid"
Theme="Office2k7Black"
Background="Transparent"
InitializeRecord="ApprovalGrid_InitializeRecord"
Style="{DynamicResource PerformantGrid}"
DataSource="{Binding StagingChanges}">
<igDP:XamDataGrid.Resources>

<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary
Source="pack://application:,,,/GLM.Infrastructure.View;component/Themes/XamDataGrid.xaml" />
</ResourceDictionary.MergedDictionaries>

<Style x:Key="DetailsCellValuePresenter" TargetType="{x:Type igDP:CellValuePresenter}"
BasedOn="{x:Static Themes:DataPresenterGeneric.CellValuePresenter}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type igDP:CellValuePresenter}">
<Button Content="View Details"
HorizontalAlignment="Center"
VerticalAlignment="Center"
Command="{Binding DataItem.ShowDetailsCommand}"
CommandParameter="{Binding DataItem}"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

<Style x:Key="CheckBoxPresenter" TargetType="{x:Type igDP:CellValuePresenter}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type igDP:CellValuePresenter}">
<CheckBox
HorizontalAlignment="Center"
IsChecked="{Binding DataItem.IsSelected, Mode=TwoWay}"
VerticalAlignment="Center"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

</ResourceDictionary>
</igDP:XamDataGrid.Resources>

<igDP:XamDataGrid.FieldLayoutSettings>
<igDP:FieldLayoutSettings
HeaderPrefixAreaDisplayMode="FieldChooserButton"
AllowAddNew="False"
SelectionTypeRecord="None"
AllowDelete="False"
AutoGenerateFields="False"
RecordSelectorLocation="None"
HighlightAlternateRecords="True"
ReevaluateFiltersOnDataChange="True"
SupportDataErrorInfo="RecordsAndCells"
DataErrorDisplayMode="ErrorIconAndHighlight"
DataRecordCellAreaStyle="{StaticResource HighlightedDataRecordCellAreaStyle}"
ExpansionIndicatorDisplayMode="CheckOnDisplay"
HeaderPlacement="OnTopOnly"
HeaderPlacementInGroupBy="OnTopOnly"
CopyFieldLabelsToClipboard="True"
AllowClipboardOperations="All">
</igDP:FieldLayoutSettings>
</igDP:XamDataGrid.FieldLayoutSettings>

<igDP:XamDataGrid.FieldSettings>
<igDP:FieldSettings
ExpandableFieldRecordExpansionMode="ExpandAlways"
ExpandableFieldRecordHeaderDisplayMode="NeverDisplayHeader"
LabelTextWrapping="NoWrap"
AllowEdit="False"
AllowRecordFiltering="False"
LabelClickAction="SortByMultipleFields"
CellValuePresenterStyle="{StaticResource CellValuePresenterStyle}"/>
</igDP:XamDataGrid.FieldSettings>

<igDP:XamDataGrid.FieldLayouts>

<igDP:FieldLayout IsDefault="True">

<igDP:FieldLayout.SortedFields>
<igDP:FieldSortDescription FieldName="LocalRequestTime" Direction="Ascending" />
</igDP:FieldLayout.SortedFields>

<igDP:FieldLayout.Fields>

<igDP:UnboundField
Name="SelectBox"
Label="">
<igDP:UnboundField.Settings>
<igDP:FieldSettings
CellMaxWidth="40"
LabelMaxWidth="40"
CellValuePresenterStyle="{StaticResource CheckBoxPresenter}"/>
</igDP:UnboundField.Settings>
</igDP:UnboundField>

<igDP:Field
Name="ChangeId"
Label="Change Id"
Visibility="Collapsed">
<igDP:Field.Settings>
<igDP:FieldSettings
CellWidth="61"
LabelWidth="61">
</igDP:FieldSettings>
</igDP:Field.Settings>
</igDP:Field>

<igDP:Field
Name="Requester"
Label="Requester">
<igDP:Field.Settings>
<igDP:FieldSettings
CellWidth="80"
LabelWidth="80"/>
</igDP:Field.Settings>
</igDP:Field>

<igDP:Field
Name="LocalRequestTime"
Label="Request Time">
<igDP:Field.Settings>
<igDP:FieldSettings
CellWidth="130"
LabelWidth="130"
AllowEdit="False">
<igDP:FieldSettings.EditorStyle>
<Style TargetType="{x:Type Editors:XamTextEditor}">
<Setter Property="Format" Value="yyyy-MM-dd HH:mm:ss" />
</Style>
</igDP:FieldSettings.EditorStyle>
</igDP:FieldSettings>
</igDP:Field.Settings>
</igDP:Field>

<igDP:Field
Name="Region"
Label="Region(s)">
<igDP:Field.Settings>
<igDP:FieldSettings
CellWidth="80"
LabelWidth="80">
</igDP:FieldSettings>
</igDP:Field.Settings>
</igDP:Field>

<igDP:Field
Name="ClientSummary"
Label="Client Summary">
<igDP:Field.Settings>
<igDP:FieldSettings
CellWidth="220"
LabelWidth="220">
</igDP:FieldSettings>
</igDP:Field.Settings>
</igDP:Field>

<igDP:Field
Name="Comment"
Label="Comment">
<igDP:Field.Settings>
<igDP:FieldSettings
CellWidth="240"
LabelWidth="240">
</igDP:FieldSettings>
</igDP:Field.Settings>
</igDP:Field>

<igDP:UnboundField
Name="Details"
Label="Details">
<igDP:Field.Settings>
<igDP:FieldSettings
CellWidth="70"
LabelWidth="70"
CellValuePresenterStyle="{StaticResource DetailsCellValuePresenter}">
</igDP:FieldSettings>
</igDP:Field.Settings>
</igDP:UnboundField>

<igDP:Field Name="ImpactedLimits" />

</igDP:FieldLayout.Fields>
</igDP:FieldLayout>

<igDP:FieldLayout ParentFieldName="ImpactedLimits">

<igDP:FieldLayout.FieldSettings>
<igDP:FieldSettings AllowRecordFiltering="False"/>
</igDP:FieldLayout.FieldSettings>

<igDP:FieldLayout.Settings>
<igDP:FieldLayoutSettings>
<igDP:FieldLayoutSettings.DataRecordPresenterStyle>
<Style TargetType="{x:Type igDP:DataRecordPresenter}"
BasedOn="{x:Null}">
<Setter Property="Margin" Value="20,0,0,0"/>
</Style>
</igDP:FieldLayoutSettings.DataRecordPresenterStyle>
</igDP:FieldLayoutSettings>
</igDP:FieldLayout.Settings>

<igDP:FieldLayout.SortedFields>
<igDP:FieldSortDescription FieldName="Schema" Direction="Ascending" />
</igDP:FieldLayout.SortedFields>

<igDP:FieldLayout.Fields>

<igDP:Field
Name="Schema"
Label="Schema">
<igDP:Field.Settings>
<igDP:FieldSettings
CellWidth="200"
LabelWidth="200"/>
</igDP:Field.Settings>
</igDP:Field>

<igDP:Field
Name="LimitId"
Label="Limit">
<igDP:Field.Settings>
<igDP:FieldSettings
CellWidth="90"
LabelWidth="90"/>
</igDP:Field.Settings>
</igDP:Field>

<igDP:Field
Name="Region"
Label="Region">
<igDP:Field.Settings>
<igDP:FieldSettings
CellWidth="200"
LabelWidth="200"/>
</igDP:Field.Settings>
</igDP:Field>

<igDP:Field Name="Clients" />

</igDP:FieldLayout.Fields>
</igDP:FieldLayout>

<igDP:FieldLayout ParentFieldName="Clients">

<igDP:FieldLayout.FieldSettings>
<igDP:FieldSettings AllowRecordFiltering="False"/>
</igDP:FieldLayout.FieldSettings>

<igDP:FieldLayout.Settings>
<igDP:FieldLayoutSettings>
<igDP:FieldLayoutSettings.DataRecordPresenterStyle>
<Style TargetType="{x:Type igDP:DataRecordPresenter}"
BasedOn="{x:Null}">
<Setter Property="Margin" Value="20,0,0,0"/>
</Style>
</igDP:FieldLayoutSettings.DataRecordPresenterStyle>
</igDP:FieldLayoutSettings>
</igDP:FieldLayout.Settings>

<igDP:FieldLayout.SortedFields>
<igDP:FieldSortDescription FieldName="Name" Direction="Ascending" />
</igDP:FieldLayout.SortedFields>

<igDP:FieldLayout.Fields>

<igDP:Field
Name="Name"
Label="Client">
<igDP:Field.Settings>
<igDP:FieldSettings
CellWidth="240"
LabelWidth="240"/>
</igDP:Field.Settings>
</igDP:Field>

<igDP:Field
Name="TNL"
Label="TNL">
<igDP:Field.Settings>
<igDP:FieldSettings
CellWidth="90"
LabelWidth="90"/>
</igDP:Field.Settings>
</igDP:Field>

</igDP:FieldLayout.Fields>
</igDP:FieldLayout>

</igDP:XamDataGrid.FieldLayouts>

</igDP:XamDataGrid>

Thanks in advance

Shahin