It will be best if you can balance cut and fill so that you don't need to import or haul away any soil. My example will be rectangular, but the same principle would apply to any shape. The odder it is, the more difficult it will be to make the calculations for establishing the elevation, but likely, you'll be able to get pretty well in the ballpark. If you have a little soil left over or run short, you'll probably find ways to deal with it pretty easily.

Imagine a curb configured in a rectangle, like a picture frame. See figure A. Next, lay the 'picture frame curb' down such that it is lying flat on the ground. See figure B. Given that the ground is pitched, in order to allow the finished area to remain level, excavate the higher elevation portion and place all the cast off soil in the area confined by the lower elevation portion of the 'picture frame curb.' See figure C. If the ground happens to be pitched more ... enough that the curb running level won't sufficiently confine the cut or fill slope, you'll have to "stack" two 'picture frame curbs' together, combining their vertical thickness in order to make a low picture frame retaining wall'. See figure D. It would continue building in the same manner for greater degrees of slope.

Keep in mind that you're not going to want the "box" created by the 'picture frame' to be completely full of soil. You'll need it to be a few inches lower in order to have room for a layer of mulch. It's possible that in order to balance cut and fill, you may need to set the overall structure slightly lower and add a little more retention at the cut end. See figure E.

You don't give the age of the kids, but beware that if they are very young you don't want the bottom end, see Yaaardvarks 'D' above, to be too high.