We use both a tag-a-long and a Santana tandem. The tag-a-long pulled behind a single seater is a real 'wagger' to handle, but oddly stabilizes a bit when hitched to the tandem. Regardless of what its behind, I really don't like the tag-along. As a child grows, get them off it ASAP. Add weight to those things and they make for an unpleasant time.

If its just you and child, setting up a comfy stoker is the way to go. Many decent tandems can be found used for $1000 and riser kits for $50. And if you like to push a little harder, mount some motorcycle accessory folding footpegs for the stoker. They can rest their feet up while you crank away. Rig it up and give them a mirror and bell too.

Other: Out of the budget but for awhile considered a triple rider. For a couple with a child and if you want to cover some miles with speed, the compromise is tandem - adult w/ child stoker and one of the adults ride another bike solo.

edit: If I had to do it again.... spotted this nice rig on fleabay. Includes stoker riser kit >
http://www.ebay.com/itm/Vintage-Kuwa...item2a4375b732