It might be easier for children to understand balancing forces when you use the example of a see-saw. Children will position themselves at different points on the see-saw to balance each other. A heavier child will sit nearer to the middle of the see-saw to balance a lighter child. The see-saw acts as pivot, so a child twice the weight of the second child will have to sit at a point half the distance from the centre compared to the lighter child. The idea of gravity can be a difficult thing to explain.
Gravity is a force which every object exerts on every other object.
When the object is small the force of gravity it has is also small. Children obviously learn very early on that things fall to the ground but it took the genius of Newton to explain why this happens. There are several important things to realize about gravity and Forces 1 will give you more detail. One very important point is that gravity decreases with distance from the object - this is why astronauts experience weightlessness. Not only that but gravity decreases with the square of the distance so the rate at which it decreases increases rapidly with distance.