OR in XAML directly:
<Button> <TextBlock>Two<LineBreak/>Lines</TextBlock> </Button>
I prefer this way:
<Button Width="100"> <TextBlock TextWrapping="Wrap">This is a fairly long button label</TextBlock></Button>
it worked for me.
Answer is very simple. Just use 
 to introduce line-break, i.e.:


<Button Content="Row 1 Text 
 Row 2 Text"/>