Every now and then I do "math" with my son (5 years now).
I just use a terminal and "bc". He loves to type in numbers and + or * and see the result. And for + we "guess" ahead of time what the result will be.
What amazed me early on: bc has variables. I explained it to him and somehow he immediately got it. You say A is 5 and B is 1 then A + B is 6.
His attention span is usually around 5 minutes, but each time it's fun.