If strapped in securely, and parent happy, why not? I have also flown two small children (with parent's permission) in one seat of a T21 (and one is now an International Comps winner, so it can't have done any harm).
However, we are back in thread drift. There are two main issues, one: how did the aircraft end in an unairworthy state, and two, where does ATC gliding go in the future.
Regarding airworthiness, it is extremelyunlikely that any of the fleet would have put anyone in any danger if given a good DI, pulled out to the launch point and flown. However, the paperwork was incapable of proving that, and a range of people at all levels, but primarily those with the responsibility for oversight are at fault. The ATC fleet was not alone in this regard, and MILCam are trying to ensure it doesn't happen in the future. IMHO, the danger is some unnecessary concentration on trivia at the expense of real engineering knowledge.
Regarding the future, there is, I fear, some nostalgia for a bygone age. Ed Meddings, who was on the team at Detling which evaluated the Cadet Mk3 prototype and pronounced it ideal told me he felt guilty for years afterwards at the torture he had inflicted on a generation or two of instructors. Times and youth have changed in attitudes and other optional activities. The fact the ATC gliding is continuing in any form is a miracle - I wish it the best of luck.