Pass data child->parent only when parent presses button - performance

Question

I have a UWP app using MVVM Toolkit. I have a parent ViewModel and a child UserControl.
Everyone says I should pass data from child to parent using Observer pattern. Which is good. MVVM Toolkit provides some classes and methods Send/Receive.
Question
How do I pass data from child user control to parent? The child has 10 text boxes and the parent needs that data after clicking a button 'Create File'. I cannot use Observer Send method after each key press in a text box.
MainPage.xaml
<Grid>...
<uc:MetadataUserControl>
</uc:MetadataUserControl>
<Button Content="Create final"></Button>
</Grid>
MainPageViewModel.cs
private MainPageModel _mainPageModel = null; // Business layer model
public MainPageModel MainPageModel
{
get { return _mainPageModel; }
set { SetProperty(ref _mainPageModel, value); }
}
void Receive() { .. } // Receive data from user controls
MainPageModel.cs
public MetadataModel {get; set;}
MetadataViewModel.cs
private MetadataModel _metadataModel = null; // Business layer model
public MetadataModel MetadataModel
{
get { return _metadataModel; }
set { SetProperty(ref _metadataModel, value); }
}
void Send() {} // Notify parents subscribed to this user control
MetadataUserControl.xaml
<Grid>
<!-- First textbox -->
<TextBlock Text="First textbox"></TextBlock>
<TextBox Text="{x:Bind Path=ViewModel.MetadataModel.FirstText}"></TextBox>
<!-- Second textbox -->
<TextBlock Text="Second textbox"></TextBlock>
<TextBox Text="{x:Bind Path=ViewModel.MetadataModel.SecondText}"></TextBox>
...
</Grid>
SecondMainPage.xaml
<Grid>...
<uc:MetadataUserControl>
</uc:MetadataUserControl>
<Button Content="Create final"></Button>
</Grid>

Answer 1

Call WeakReferenceMessenger.Default.Send(new NotifyParentMessage(new YourInfoClass())); in your Send() method in the child's view model and make sure that the YourInfoClass class contains all information that the parent view model needs about the child, i.e. the values of all properties that are bound to the textboxes in the view.

Answer 2

Managed to do it using Messenger's request feature. When the parent sends an event, the children replies with the object required.
MainViewModel.cs
private void CreateFile()
{
// Request data from children user controls
var metadataRequested = WeakReferenceMessenger.Default.Send<MetadataRequestMessage>();
}
MetadataViewModel.cs constructor
WeakReferenceMessenger.Default.Register<MetadataViewModel, MetadataRequestMessage>(this, (r, m) =>
{
m.Reply(r.MetadataModel);
});
..
public class MetadataRequestMessage : RequestMessage<MetadataModel>
{
}