I mentioned previously that I designed some stacking number blocks for my grandson (on thingiverse here). He loves them, and knows the numbers and the order now. Technically you can sort of stack the 10 on top of the 6 instead of the 9, but not the right way up or even remotely "square", but otherwise it works very well - you can only stack in the right order, which is the idea!
Now I have some new plastic, I have been able to print them in the proper colours: Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Grey, and White.
Let the learning begin :-)