Nice work. I like the simple, unobtrusive and ad-free interface. I have kids who could use this although you face lots of competition for their very limited allotted active screen time [1]. The 6 yo gets 45 mins a week on the computer, usually opting for PBSkids, Kodable or Scratch. The 3 yo gets 15 mins usually opting for guided digital canvas painting as she learns to use a mouse.
There are a few good no-screen puzzle books for kids (lookup the ones by Usborne).
As a recent publisher of an interactive children's book [2], I am seeing more sales of the physical books than the digital/web editions despite the latter having more features including an element of teaching math.
Curious if you considered publishing a physical book with these concepts/math puzzles?
[1] This app inspires me to upgrade some of their passive screen time (TV) to more of your and similar apps