Let’s assume you have a <div> element with 20 <a> tags inside, and each <a> tag has a <span> somewhere inside it. If you’d like to access all those spans you should use this jQuery line:

$('div a').children(':first-child')

Common mistake is to query like that:

$('div a:first-child')

but that is completely other thing, and will return the first <a> tag, not the <span> inside all of them.