const text = '|||';
.$el.children().slice(1).before(text);
// или
$el.children(':nth-child(n + 2)').before(text);
// или
$el.find('> :nth-last-child(n + 2)').after(text);
Array.prototype.forEach.call(
el.children,
(n, i) => i && el.insertBefore(document.createTextNode(text), n)
);
// или
for (const n of el.children) {
if (n.nextElementSibling) {
n.insertAdjacentText('afterend', text);
}
}