How was the loop like heaven and a prison at the same time?

The loop was like Heaven because it was "beautiful and good', it was also like a prison because once there..... the children could never leave.

Yes, it was beautiful and life was good, but if every day were exactly alike and if the kids really couldn’t leave, as Miss Peregrine had said, then this place wasn’t just a heaven but a kind of prison, too. It was just so hypnotizingly pleasant that it might take a person years to notice, and by then it would be too late; leaving would be too dangerous.