The first thing to communicate to kids is that gravity is something that works because it affects the very fabric of space. To illustrate this idea, use a trampoline or other stretchy sheet. Place some balls in the corner of the trampoline and have one or more of the students stand in the center. The balls will roll toward them. Explain that smaller objects move toward larger objects because they cause this bending in space, which makes things roll toward them down the slopes created in the fabric of space.
You need to explain that the weight stretches space inward in all directions, not just downward. If students don't see this, try using two sheets of clear acetate. Place the sheets on either side of a ball and press them in around it, making a bulge on each side. Turn the sheets and ball at various diagonals and trickle a colored fluid of some sort in between the sheets at the top edge. Regardless of which way you turn it, the fluid will be pulled in toward the ball.
The explanation given, using the stretchy sheet, is usually sufficient to explain why we and other things are drawn toward Earth, but it doesn't explain yet why gravity keeps things in orbit rather than just dragging them to the heaviest object. To make this clear, you need to explain that in space an object that is moving will not be slowed down by air resistance and will go on forever. When a moving object passes another object, it is pulled toward it but retains its momentum. Its path gets curved, but it does not stop and turn directly toward the heavy object. If the two forces are sufficiently matched, then the path of the object will be curved into an ellipse, and the object will keep going around in orbit. Explain this with the trampoline again. Have the child stand in the middle and roll a ball past them across the slope made by their weight. They will see that the path is not straight but curves in around the weight. It will eventually crash into the child because its momentum is exhausted by friction, but this would not be the case in space. You can either explain this without a prop or go a step further and get a remote-controlled car. Drive it forward along the trampoline, again past the child. Again, its path will be curved, even though you are only driving straight forward. If you can get the speed just right, it will form an orbit.
If the students understand this explanation, introduce the basic idea of gravity bending time in addition to space. Start with an explanation of a car driving along. The two wheels on the right side go into thick mud while the wheels on the outside stay on the asphalt; what happens? They will probably see that the car turns toward the right side because the wheels on the right are going slower. Then explain that gravity actually causes time to slow down, and the stronger it is, the more the slowing will happen. When an object passes by another heavy object, the side closest feels more gravity than the further side. For this reason it slows down more and turns toward the gravity well, just like the car. That will get them thinking.