It's a bit awkward since "poisonous" usually refers to things you eat, and sandboxes or their contents are generally not eaten.
If you want to continue the sandbox metaphor, which I do like, "playing in the Fed's sandbox until the bottom fell out" might work. Or even "toxic sandbox".
If you want to continue the sandbox metaphor, which I do like, "playing in the Fed's sandbox until the bottom fell out" might work. Or even "toxic sandbox".