Kids tend to confuse force and energy. This lesson is to clarify the distinction without getting into math.
Force is simply a push or pull (without reference to where or how that pull or push is generated). For example, in sitting on a table you are applying a force down (your weight on the table) and the legs of the table are are applying the same force up. Note that a force (a push or pull) or multiple forces can be simultaneously applied in any direction.
The fact that an object does not move says that forces acting on it are equal in all directions. If forces on an object are greater in one direction than another the other, the object will move accordingly.
With your kids, consider any object near you and analyze forces acting on it that keep it stationary. For example, take sitting on a table again. If its legs are incapable of exerting equal force up, they would break and you would come crashing down. If legs were pushing up with grater force than you were pushing down, the table with you on it would rise into space. (Please post how this goes with your kids and their questions.)