You could control the spacing among children of the box control. For example, the following example puts 5em at both the upper margin and the lower margin. Notice: the total space between two input fields is 10em.

<vbox spacing="5em">
    <textbox/>    
    <datebox/>    
</vbox>

Another example illustrated an interesting layout by use of zero spacing.

<window title="Box Layout Demo" border="normal">
    <hbox spacing="0">    
        <window border="normal">0</window>        
        <vbox spacing="0">        
            <hbox spacing="0">            
                <window border="normal">1</window>                
                <window border="normal">2</window>                
                <vbox spacing="0">                
                    <window border="normal">3</window>                    
                    <window border="normal">4</window>                    
                </vbox>                
            </hbox>            
            <hbox spacing="0">            
                <vbox spacing="0">                
                    <window border="normal">5</window>                     
                    <window border="normal">6</window>                    
                </vbox>                
                <window border="normal">7</window>                
                <window border="normal">8</window>                
                <window border="normal">9</window>                
            </hbox>            
        </vbox>        
    </hbox>    
</window>