function startCalc(){
  interval = setInterval("calc()",1);
}
function calc(){
  one = document.searchFormjReviews.jr_volwassenen.value;
  two = document.searchFormjReviews.jr_kinderen.value; 
  document.searchFormjReviews.jr_personen.value = (one * 1) + (two * 1);
}
function stopCalc(){
  clearInterval(interval);
}