The new event model is much better. There is a WindowCreated event that fires when a tool window is first created. A WindowInitializing event fires before the tool window is first activated. This event allows you to lazy initialize the tool window, meaning initialize it only as needed so that you don't initialize tool windows that aren't going to be displayed.
There is no more need for a Child property since now, tool windows (ToolWindow class) are Controls themselves. You simply add child controls directly to the ToolWindow control. This eliminates the need for UserControl wrappers like we had in DockableWindow.
Right now there isn't a FocusedToolWindow property. I'll add that to the TODO list. There is a WindowFocused event that tells you whenever a window gains focus. The old GetFocusedToolWindow method in DW simply looped through the tool windows and checked to see if one contained focus.