Why not build your own vise grip clamps? No reason I can think of. This one was built using 1/2 and 3/4 inch Baltic birch plywood. There is a free template of the parts you will need to cut out as well as full instructions on the build.
There are just three parts to this kids knockdown stool, so it is easy to build and assemble. This one was build using scrap plywood and you can find the free templates and instructions at the link.