make believe

make believe (to pretend or imagine) — притворяться; изображать из себя (детск.)

Example: The kids were making believe they were cowboys.

make-believe {adj.} (imaginary; not real)

Example 1: We’re playing a game. He’s a make-believe doctor and I’m a make-believe patient.
Example 2: He's always in this world of make-believe, and he's never in the world of reality.

Related vocabulary:
make like… {AmE, infml} to pretend to be, know or have something in order to impress people

Example: He makes like he's the greatest actor of all time. (OALD)

Example 2: She makes like she’s the boss round here. (OALD)

make as if to do sth (to make a movement that makes it seem as if you are just going to do something) — сделать вид, что...; притвориться, что...

Example 1: He made as if to speak. (OALD)

Example 2: Dad made as if to chase me out of the room. (MM)