If the child leaves a blank space, praise them. It shows they recognize a difficult rule rather than guessing blindly.
Exercises emphasize the difference between (what we hear) and letters (what we write). proveriaem domashnie zadaniia soloveichik 1 klass
There are specific manuals for the "Harmony" (Гармония) program that explain the pedagogy behind the exercises. If the child leaves a blank space, praise them