public class ChildFormController extends SimpleFormController implements InitializingBean
{
...

/**
* This method is invoked to see if the form command has been modified (without the need for final submission)
*/
@Override
protected boolean isFormChangeRequest(HttpServletRequest request)
{
// return true if the addChild is not null
return ( request.getParameter( "addChild" ) != null );
}

/**
* This method is invoked if the 'isFormChangeRequest' returns true
*/
@Override
protected synchronized void onFormChange(HttpServletRequest request, HttpServletResponse response, Object command) throws Exception
{
Child child = ( Child ) command;

if( request.getParameter( "addChild" ) != null )
{
// add a new child item
...
}

...
}