So here we are....how did plesiosaurs become extinct?
The devastation caused by the giant meterorite that hit Mexico also destroyed ecological systems all over the earth, including those in the seas. Also this was a period of massive volcanic activity. and this series of massive eruptions around 60-65 million years ago, and may have contributed to the mass extinction event. Volcanic eruptions produce large amounts of toxic gasses, and these can also have a devastating effect on living organisms.
It is likely that both these event contributed to the wide-scale destruction of habitats both on land and in the water.
Geez, why can't "they just died ok?" not be an answer. He's only 4.
And that dinosaur box in the photo is absolutely chocker block full of dinosaurs.