Teoria.com website is free and excellent. https://www.teoria.com/en/exercises/
May the internet work for us, not against us, in these trying times. I strive in my personal teaching to be as engaging and addicting as online games, but now that we’re all stuck in our screen-ridden homes, you and your child can experience the real deal!
Teoria is a western classical music theory learning resource that offers both tutorials and interactive exercises. The tutorials will be incomprehensible to younger kids, but parents might benefit from the overview. The ear training exercises, on the other hand, are appropriate for all ages.
One of my favorite games is the “interval from random note” quiz: https://www.teoria.com/en/exercises/ie.php
Intervals are the distances between pitches. These relationships are the basic building blocks of harmony and melody. The upper left quadrant of the options screen allows you to toggle on and off the various intervals, so that you can start with only two and build up to all twelve within the octave. If you are confused about what any of this means, consult the tutorial section or your teacher.
The chord quiz is also great. At the simplest level you can learn to distinguish between major and minor triads: https://www.teoria.com/en/exercises/c3e.php
I still use this website frequently. Go forth and have fun!