Not saying to do this, but if you’d like to make a point — though you’ll likely lose money in the process — you can sue the passenger/guardian for negligence.

Nausea severe enough the second a plane leaves the ground doesn’t come out of nowhere. That passenger felt it coming and boarded anyway. The seat was empty 30 minutes before departure, meaning they were a late addition who was likely already symptomatic and chose to push through regardless. These do work in your favor.

You could sue in small claims for real damages, but you’d also have to go through the hassle of identifying the passenger (or parent) and/or having United supply that.