Terrence kept an eye on Fletcher as he worked, setting up wards around the park. It didn't take him long to create a Muggle-repelling ward, keeping their targets within the boundaries; they would probably do some amusing bouncing back from the invisible barrier if they tried to escape. It wasn't a ward Terrence used often - most of his practice was with hiding his work in the Department of Mysteries, and there certainly weren't any Muggles in there - but it was much simpler than what he normally did.
He noticed that Fletcher was getting rid of the kids, and even though he would have gone along with it either way, he was slightly pleased by that. The man did have some principles, maybe - not morals, who gave a fuck about morals, but personal rules were good - and that made Terrence like him more.
Although, they were probably about to kill those children's parents. It was definitely debatable whether that was better or worse. But in support of the decision, he made the barrier impermeable from the outside as well, so no kids would wander back in and get hit by their flying arrows.
He rejoined Fletcher as he was finishing up painting, and grinned at his handiwork. "Nice," he said. When the other man was done, he held out one of the bows. "Ready when you are!"