I'd use a large aperture, and rather than put the child immediately in front of the tree, I'd put quite a distance between the child and the tree. (as a general guideline, I almost never put a subject immediately in front of any background... I want a LOT of space. I make exceptions to this, but more often I find a background looks better if it's "in the background" and not immediately behind the subject.
Here's an example:
Holiday Bokeh by
Tim Campbell1, on Flickr
Ok, it's not a child (but that shouldn't matter). This shot was taken with a 70mm focal length at f/2.8 and there's at least 10' (probably closer to 15') between my subject and the tree.
Deliberately de-focused holiday lights create beautiful orbs of soft light. You need a low focal ratio (preferably at least f/4 or lower) and a long-ish focal ratio (this works at 70mm... at 50mm it'd still be pretty good. But if we dropped to, say, 18mm then we would lose most of this effect.)
The result is a sharp subject, but the tree gets soft holiday light bokeh. Not only is it not necessary for the Christmas tree to be sharply focused, being out of focus gives it even more beauty. It's clear to everyone that the background is a Christmas tree.
I also toyed with the white balance and while this is FAR from being correctly white balanced, I decided the "warm" look of tungsten light helped create a warmer holiday look. (I don't always want "accurate" white balance.)