This 12" high child's wood stool is easily made using stock dimensional lumber (standard or closest metric size) all cut at 90 degrees and assembled with glue and nails/screws. The overlapping joint technique is simple, direct, and strong. It's versatile and adaptable to many designs -- see my blog for lots of other stuff. http://furnituredesignbank.com
Use this exploded view to create your cutting list. You can make this stool longer, more bench like, or even deeper by using different stock lumber seat planks -- like1x3's for example, and then there's plywood or anything handy and recyclable -- it's the overlapping joint that counts.