a/20818874/1636522

HTML

<div id="pa" style="background-color:red">
    <div style="display:inline">one</div>
    <div style="display:inline">two</div>
    <div style="display:inline">three</div>
</div>

JavaScript

document.getElementById('pa').onclick = function (e) {
    var i = 0, tgt = e.target, items;
    if (tgt === this) return;
    items = children(document.getElementById('pa'));
    while (tgt.parentNode !== this) tgt = tgt.parentNode;
    while (items[i] !== tgt) i++;
    output('index : ' + items[i].innerHTML);
};

function children(el) {
    var i = 0, children = [], child;
    while (child = el.childNodes[i++]) {
        if (child.nodeType === 1) children.push(child);
    }
    return children;
}

function output(s) {
    alert(s);
}