I use a Serenity Net tent under my Trailstar (good fit, not quite perfect though with a rectangle under a 5-sided 'mid). I have plenty of room, especially with the newer version with raised corners which raises the angled ceiling panels a few inches. It is much better than a cramped bivy in milder weather IF you have to spend any time in a shelter, like getting to camp hours earlier than bedtime. And its light enough that it can be carried but not used if not needed.
It is what it is, though; a UL solo inner, and really too tight for two, even a child (maybe especially with a child. They have that nasty habit of turning sideways and pressing their feet into your back).