playing dead; planking
A children's game or internet meme where participants lie still as if dead; 'planking' refers to the stiff, face-down pose popularized online.
The children were playing dead in the park.
That photo is a type of playing dead, called planking.
Compound of 死体 (corpse) and ごっこ (pretend play). The exact origin is uncertain, but it likely emerged from children's games and later gained internet meme status.