MUST-HAVE APPS and GAMES
Other than the must-apps like Wolfram Alpha, Geogebra, Desmos for every math teacher and student, there are topic-specific apps and some good games I can recommend too.
Kids can play some of those games without even realizing their mathematical connections like "tangled" for the introduction to graph theory or "cube connect" for 3 dimensional thinking. There are some apps that serves the same purpose, both powerful, but their user profile is different.
As a teacher or a parent-, if you would like to teach about 'the cross sections of a 3D solid', you may want to use a more professional app like "icrosss", but as a kid if you want to practice, I'll definitely suggest "crafty cut" because of its more game-like nature.
Apart from these, as a parent or teacher, most probably you have already realized that, there are more than 200 000 educational apps and even more 'lists for the top educational apps'. So it is really hard to choose, thats why here, I'll only include the ones that I have used and liked its content.