Pool "walking

[ga1951]

We just purchased a small pool for the 4 kids, ages 9,7,5,3. This is the type of pool that expands as the water is filling it, with a filter and ladder. We've leveled it with sand, but a border around it and it seems to "walk" as the kids are in playing. Anyone else have this problem or know what to do to correct it??? It's only 13' x 36" deep, just big enough for little ones. I'm not feeling real safe with it shifting the way it does.