I went from trailer to trail-a-bike to tandem with my son. As he got bigger, the trail-a-bike was hard to balance and the tandem was less of a problem in that regard. We went with a Trek T900 which has a sloped top tube that let the 8 (now 9) year old ride without any mods other than changing out the suspension seatpost for a regular one dropped to the lowest position.
We do go on grocery runs with this rig (panniers). And when he's properly motivated and pedaling we can really get moving once we overcome the weight of the bike and load.