I was looking around at my kid's blocks, and sure, there were plenty squares, circles, and triangles. I wanted to add some variety to those standard shapes, so I tossed these together. They're 40mm tall and are at least 25mm or so wide at their most narrow (i.e., NOT a choking hazard). The edges are rounded as well, so hopefully a kid won't stab their eyes out. No promises on that. Kids will be kids.
See time-lapses of the things I print here: https://www.youtube.com/channel/UCQ9nW8XQAPWKGGKRd9LFQNQ
I included a separate STL file for each block/shape if you want to print different colors, or if you're limited on build platform area. For convenience, I also included a ZIP file (Complete-Set.zip) of the individual pieces.
For those with the build platform real estate, there is an STL file (Complete-Set.stl) that has all of the pieces in one file.
I've also included the original 123D file if you want to play with the designs.