chkb = ff_getElementByName(chkbNaam);
var labelId = "bfLabel" + chkb.id.split("ff_elem");//this split gives an array with the last element being the identnumber which is the same in the label object
var chkbLabel = document.getElementById(labelId);
chkbLabel.textContent = arrKl[r];
It was actually straightforward navigating through the DOM. So look at the structure in my picture of the HTML. When I select the checkbox I only get the object with the type checkbox. However, this checkbox has an id of ff_elemxxxx and is accompanied by a label with id =bfLabelxxxx. So select this element by id and you can change the label text by changing the property textContent.