How to add CSS if element has more than one child?

You can't directly 'count' total numbers of elements in CSS, so there's no way to only apply the class if there's 2 or more divs (you'd need JavaScript for that).

But a possible workaround is to apply the class to all divs in the td...

td > div {
    margin-bottom: 10px;
}

... and then override/disable it with a different style when there's only one element. That indirectly lets you add the style when there's 2+ more child elements.

td > div:only-child {
    margin-bottom: 0px;
}

Alternatively you can apply to every div after the first one, if that happens to work for your situation.

td > div:not(:first-child) {
    margin-bottom: 10px;
}

Edit: Or as Itay says in the comment, use a sibling selector

td > div + div {
    margin-bottom: 10px;
}

td > div:not(:only-child) { margin-bottom: 10px; }