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".
In my experience that is not a correct statement for the target demographics.
poisonous (adj.)
[...]
2. (figuratively) Negative, harmful.
Synonym: toxic
Poisons are something you do to yourself (usually by ingestion), venoms are injected by the other organism (such as a snake bite).
The Caddyshack Baby Ruth scene seems apropos.
Fed Floaters. Everybody out! Pool's closed.
All toxic substances are poisonous. A toxin is a poison produced by a living cell or organism.
Poisonous Sandbox was elegantly simple, pithy even, and quite appropriate.
"Toxin" is a newer invention, derived from "toxic" by adding the biochemistry-related suffix "-in" to indicate toxic substances of biological origin.