A single trip for air experience/trial lesson in a glider is about as safe as gliding gets - no unusual attitutes, usually within the circuit or not far away, no manoeuvres likely to cause a problem.
It is not totally safe - no life experience is (there was once a fatality when someone was playing chess!). It is statistically less dangerous than powerflying, but I know of one fatality which did occur during a trial lesson - one in perhaps hundreds of thousands. (New instructor, spun in from final turn in a glider with known sudden departure characteristics - but the instructor, having been told about that, didn't believe it or that it could happen to him.)
There has been one bail-out with a first-timer on board - successful - when a freak lightning strike hit a glider and literally blew it apart. The first-timer had been thoroughly briefed on parachute and bail-out procedure "just in case" and it all worked. Better chance than in a light a/c where parachutes are uncommon.
There have been a few minor injuries from heavy landings. Shouldn't happen with competent instructors, but occasionally has.
Check out with the club you hope to fly from what is their accident record, what experience will the instructor have, do they give proper briefing etc. - all should say the right sort of things.
In the end, though, people who go gliding are not pax in an airline - they join the gliding club as members, and fly as a member of the flight crew (Pilot under training). It is a sport, not a passennger ride, even if they don't want to touch the controls.
Chris N.