Forces work in pairs. The force that moves the object and the force that works against the object.
An object moves in the direction of the force that is applied.
An object moves in the direction of the force that is applied.

COMBINED FORCE
Both children are pulling with the same amount of force. As a result, the rope with not move.
One side needs more force in order for the rope to move. (see below)
Both children are pulling with the same amount of force. As a result, the rope with not move.
One side needs more force in order for the rope to move. (see below)

GRAVITY IS A FORCE THAT PULL OBJECTS TOWARDS THE EARTH.