What it teaches
Choosing between ą and ę. Unlike ó and u these two do not sound the same, but they swap inside a word family — ząb and zęby, dąb and dęby, gęś and gąski — and a child who reaches for the related word gets most of them right without remembering any of them.
The endings are the other reliable group: an infinitive ending in -ąć takes ą every time.
What to look for
Say the related word out loud together — "ząb, and more than one is?" — and let the child hear the swap. It works far better than the written hint.
Ą before ć is the one worth drilling separately: wziąć, zacząć, zdjąć. That group is small and it is on level three.