What it teaches
Choosing between ch and h. The one rule that never fails is the ending: a word that finishes with this sound finishes with ch.
The second is the swap: ch often turns into sz in a related word — mucha and muszka, ucho and uszy. What is left is a short list of words with h, and a short list is something a child can hold.
What to look for
Start with the ending rule alone. It is absolute, it is easy to check, and it settles a good share of the words a child writes in a week.