These functions are missing from the VirtualizingPanel class:
protected void AddInternalChild(UIElement child)
protected void InsertInternalChild(Int32 index, UIElement child)
protected void RemoveInternalChildRange(Int32 index, Int32 range)
protected override void OnItemsChanged(object sender, ItemsChangedEventArgs e)

This property is missing from the Panel base class:
protected internal System.Windows.Controls.UIElementCollection InternalChildren { get; }

Panel's InternalChildren property is just the same as Children, so you can use that.

All the Add/Insert/Remove Child methods are just redirected to the children collection, so you can use Children.Add/Insert/Remove instead.

Recommended work-around for now from Sergio:
And you can subscribe to the ItemsChanged event on the ItemContainerGenerator of the corresponding ItemsControl, something like this:

public class VirtualizingTilePanel : Noesis.VirtualizingPanel, Noesis.IScrollInfo
{
public VirtualizingTilePanel()
{
Loaded += (s, e) =>
{
ItemsControl itemsControl = ItemsControl.GetItemsOwner(this);
itemsControl?.Generator?.ItemsChanged += OnItemsChanged;
}
}
private void OnItemsChanged(object sender, ItemsChangedEventArgs e)
{
}
}