WPF UserControl with multiple Children

I have a UserControl that I would like to be able to have more than one child. It already has StackPanel as it's child, so what am I doing wrong?

Ultimately, I'd like for the control to include some of its own children automatically and then, when it's used, allow placing more children inside one of its elements. I tried MSDN's How to Override the Logical Tree without success.

Thanks in advance.

LayerPanelItem.xaml:

<UserControl x:Class="Controls.LayerPanelItem"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/wi开发者_运维问答nfx/2006/xaml">
 <StackPanel></StackPanel>
</UserControl>

Main.xaml:

<controls:LayerPanelItem>   
    <TextBlock>Test</TextBlock>
    <TextBlock>Test</TextBlock> <!-- Error:  The property 'Content' is set more than once. -->
</controls:LayerPanelItem>

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You can't do this (directly) with a UserControl. Instead, you'll need to derive from ItemsControl, and put your own custom layout logic in the subclass.

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Try to add Grid in your UserControl and add rest of child to that grid.