The Mosquito Coast - 1986
In this scene, the blond boy is eating his pop-corn but is suddenly distracted by somebody standing in front of him (member of film crew perhaps?). He then looks over at this person who appears to give him instructions and prompts him to carry out the 'scene' which appears was discussed privately beforehand between that person and the boy. The boy down puts the pop-corn bowl away and proceeds to carry out a sequence of body movements.
In the scene note the location of the boys hands and movement of his fingers while his eyes are closed. It is a simulation of masturbation.
Note how the boy arches his body into the air and flops down quickly. ('Orgasm')
Note where his eyes are focusing and his hands.