What is the burden of identifying as an officer to a minor?

'We note at the outset that the officers' identification of themselves to the girl did not constitute substantial compliance with section 844. That section requires that an officer explain his purpose before demanding admittance, not merely that he identify himself as an officer. 'The burden of making an express announcement (of purpose) is certainly slight.' (Miller v. United States, 357 U.S. 301, 309, 78 S.Ct. 1190. 2 L.Ed.2d 1332.)

Page 57