The levels present a fair challenge, and after testing them with my children (ages 9-12), I was impressed by their ability to find the minimum move order for the first two levels. Interestingly, given the choice between solving the game and doing chores, they consistently chose the game.
I even developed a solver to determine the minimum number of moves for each level. The solver efficiently handles levels 1, 2, 3, and 6, but it struggled with the remaining levels, taking the entire night without success. If you're interested, the GitHub repository [2] includes a convenient CSV format dump of all the levels if you want to give a solver a try.
One thing I could not fix is responsive design for phone and smaller desktop screen, sorry!