What it teaches
Hearing pitch, which is the foundation of everything musical and is not taught by naming notes. A child who cannot hear that one note is higher than another cannot sing in tune or follow a tune, and it is a skill that improves quickly with practice.
How it works
Two notes are played, one after the other, and the child says which was higher. The pair can be replayed as often as wanted.
The gap between the notes narrows as the levels go up: far apart at first, then closer together, which is where the ear has to work.
What to look for
This one needs sound, so it is not a game for a quiet room. Headphones help considerably, because the small intervals at the higher levels are genuinely hard on a tablet speaker.