Grow one idea
A story grows when one moment changes the next one.
You do not need to add more characters, bigger danger, or a lesson. Help the child notice what their first idea causes and let that consequence create the next page.
Ask what changes because of the idea
If a penguin opens a bakery, what becomes possible now? Who smells the bread? What rule becomes difficult? The useful question is not “Then what?” in the abstract, but “What is different because that happened?”
Move from event to choice
Events can pile up without becoming a story. A character choice creates direction. Ask what the character wants, what they are worried about, or which promise they are trying to keep. The child can then decide how the character responds to the changed situation.
End when the important change is visible
The story may be finished when the original situation has changed in a way the child cares about. The bakery has a new rule, the lonely dragon has a customer, or the character decides something they could not decide on page one.
Read the pages in order and look for the chain: this happened, so that mattered, so the character chose. If the chain is understandable to the child, the story is doing its job.