Add child to user control

[ContentProperty("PlaceHolder")]
public partial class UserControl1 : UserControl
{
    public UserControl1()
    {
        InitializeComponent();
    }

    public object PlaceHolder
    {
        get
        {
            return this.contentPresenter.Content;
        }
        set
        {
            this.contentPresenter.Content = value;
        }
    }
}

<UserControl x:Class="WpfApplication9.UserControl1"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <DockPanel LastChildFill="True">
        <Border removed="Plum" DockPanel.Dock="Top" Height="50" />
        <Border removed="Blue" DockPanel.Dock="Bottom" Height="50" />
        <ContentPresenter x:Name="contentPresenter" />
    </DockPanel>
</UserControl>