I think it's great, but my 7yo son doesn't want to use it every day, because the countdown timer creates unnecessary pressure.
I thought about sticking a piece of masking tape on the iPad screen to cover the countdown timer, but my son didn't like that idea.
So I built something similar, but without the timer: https://math.twilam.com/
The features I plan to add to make it 'complete': 1. Select operation types and number ranges (like sophiebits' site). 2. Select the number of problems you want to do (analogous to sophiebits' timer). 3. A 'power bar' which fills up as you reach the number of problems you wanted to do.
I might also add: 4. A way to persist config options. 5. At the start and end of each session, display the length (# days) of your current streak.