This is a common situation in XAML based apps (WPF/Silverlight/WinRT).
WPF related link -> WPF Databinding: How do I access the "parent" data context?
There are several ways you can deal with this issue:
ElementName binding is the most common approach, as Filip pointed out.
You could walk visual tree till you find the parent. That is what FindAcestor does internally. You could dress it up in behavior for easy reuse.
If you use view models you could use messages instead of bindings or you could add parent context to each child view model.
Picking the best solution will depend on your specific circumstances.