Wrap existing SVG elements using the svg.js library

How can I wrap existing <svg> element in svg.js?

For example:

<svg>
    <circle r="10" cx="10" cy="10" fill="red"></circle>
</svg>

I know this can be built with svg.js very easy, but I take it as example for my question.

var existingSvg = Svg.<some-method>(document.querySelector("svg"));
existingSvg.children(); // circle

How can I do this?

I tried using SVG('<id-ofsvg-element>'), but children() returns an empty array.