I answered this myself..
I added two touchstart listeners that work far better than mousedown or onclick on touch devices. My two year old had a lot of trouble pressing a button on an onclick event and a mousedown event but not on a touchstart event.
<script type="text/javascript">
var t = document.getElementById("minus");
var t2 = document.getElementById("plus");
t.addEventListener("touchstart", touchHandler, false);
t2.addEventListener("touchstart", touchHandler2, false);
function touchHandler(event) {
numMinus();
}
function touchHandler2(event) {
numPlus();
}
</script>
-Kyle