Statistically gliding is slightly less safe than certified GA flying. GA flying from 1990-1999 suffered 1 fatal per 70,000 hrs, compared to 1 per 47,000 hrs.
In a half hour trial flight, statistically then there's arguably a 1 in 94,000 chance of your children being killed, as compared to a 1 in 16,000 chance of either of them being killed on the road in a given year (based on 3,500 deaths per year and 56 million people in the UK).
However realistically, virtually all glider fatalities are due to collisions due to several gliders thermalling under the same cloud - highly unlikely to occur on a trial flight. Also inevitably, they'll be flying with a QGI who (particularly with a child on board) will be a more experienced pilot, flying safely. I'd say this probably puts the realistic odds of a fatality at about 1 in a quarter million.
Hope this helps. If you want chapter and verse on GA accidents, the CAA publish CAP 701, which costs about £6 and I regularly use to explain why flying little aeroplanes is perfectly safe.
G