The derailleur and chain on this youth mountain bike was destroyed when it came into the shop. Clearly, there was a problem with the limit screw setting that led to this disastrous (and dangerous) outcome.
The derailleur has two limit screws, The High ("H") limit screw aligns the smallest cog of the rear cassette/freewheel with the guide pulley of the derailleur. The Low ("L") limit screw aligns the largest cog and prevents derailing towards the spokes/wheel.
In this case, the L limit screw was not set correctly and the rider shifted the chain and guide pulley into the spokes, causing the derailleur to break from the hanger and deform badly.
The bike required a new chain and derailleur.
An incorrectly set L limit screw is dangerous! Always check your limit screws on new bikes and after routine maintenance has been performed by attempting to shift beyond the largest cog on your rear wheel - if the chain falls off the cassette, you have a problem.
Ride Free!