Final answer:
Sophie believed that Room 66's biggest problem was it perpetuated destructive thoughts instead of helping children resolve their emotional issues.
Step-by-step explanation:
The biggest problem in Room 66, according to Sophie, was that the room had become a place that fostered destructive thoughts rather than helping to alleviate them.
This high-tech room, initially designed to help children work through their emotional problems, had become stuck in a pattern due to the children constantly thinking about a violent scenario set in an African veldt. As a result, the room was no longer a therapeutic escape but a concerning reflection of the children's inner turmoil.