Obviously what this world needs is MORE tetromino designs. This includes 2 different sizes, including some [potentially] baby safe 40mm version. No guarantees on that though. Kids will be kids. There's even a holder for the smaller 10mm design that holds a "complete set" of 2 of each design. See the picture or this link for details: http://en.wikipedia.org/wiki/Tetromino#Tiling_the_rectangle_and_filling_the_box_with_2D_pieces.
See time-lapses of the things I print here: https://www.youtube.com/channel/UCQ9nW8XQAPWKGGKRd9LFQNQ
Technical
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 (*-CompleteSet.zip) of the individual pieces.
For the 10mm size, there is also a "holder" (10mm-Holder-CompleteSet-WithDoubles.stl) that will neatly hold a complete set of pieces if you print 2 of each size. They will fit in a 5x8 rectangle per the included picture and the link in the "Description".