It's a very concrete learning experience for them. Make sure you schedule enough time for them to be able to complete the activity in class, then sometimes I give them extra credit if they want to go home and paint it and make it more realistic. I have gotten some great topo models that I keep from year to year. I would suggest that you try doing this yourself first. The first time I did this, I picked topographic areas that were hard for me to construct.
Mountains are the easiest because they go from largest to smallest as they go up.
What I do with my little ones, and I understand it works with the bigger ones, too, is give them a good sized lump of clay. The build it into a mountain shape. I ask them to score a shallow line down the side, enough to see, but not split or damage the mountain. Then I give them a piece of fishing line and have them wrap it around about in the middle, then pull through to cut the mountain in half, but leave it together. Next, I have them cut each half in half in the same manner, leaving it four pieces stacked together.