Iterate through all divs within a div

Hi there,
I need to iterate through a Div which contains other divs.
I want to find all the ones with ID that starts with "ReportDataDivID".  The following code works, but I am iterating via all the DOM.  I want to just go over the testDiv which will be passed to this function. How can I efficiently do this.  Thank you.

var __ensureVisible = function(testDiv){
        var divs = document.getElementsByTagName("div");
        for(var i = 0; i < divs.length; i++){
            var regexp = new RegExp('^' + "ReportDataDivID");
            if(regexp.test(divs[i].id)){
                $("#" + divs[i].id).show();
            }
        }
    };

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