A panel that passes size recommendations to, but not from, children.

NannyPanel.cs

using System.Windows;
using System.Windows.Controls;

namespace MonkeyBusters
{
    public class NannyPanel : Panel
    {
        protected override Size ArrangeOverride(Size finalSize)
        {
            foreach (var child in Children)
            {
                child.Arrange(new Rect(new Point(), finalSize));
                child.InvalidateArrange();
            }

            return base.ArrangeOverride(finalSize);
        }
    }
}