I downloaded this from Google Play for my mother last week - she loves Sudoku.
Your game worked well and looked good.
In the absence of instructions I had to play around with it and figure out what to do first and then teach my mother. We missed having an eraser - even though you made it easy to replace the colours or drag them out - my mother doesn't have the control over her hands to drag the block out and sometimes you want to clear it without replacing it right that minute.
Because you're just playing the same level over and over again, even though it is randomly generated - there was no progression for her - no reward, goal or achievement. Even though you time how long it takes, we didn't see that afterwards - i.e. there wasn't a table with best times. It's not really necessary, but it is something that people like - so might be an idea.
Also because she daily plays sudoku games with 9 squares, playing with only 6 squares wasn't very challenging for her.
I'd also suggest you submit the game to the "designed for families" section in Google Play - since it's something that would appeal to the families. It then gets a badge that says it's family friendly. (It's an option you tick somewhere under Store Presence ). https://play.google.com/about/families/children-and-families/#!?zippy_activeEl=d... It's a fun game for younger children and you'll get more exposure in that section.