Find out which kind of decision this is
Ask whether the book was chosen for pleasure, borrowed for a specific question, or assigned for class. Those purposes matter. If it is optional, the child may simply dislike the story or want something different. If they cannot follow the text but are interested in the topic, shared reading may be an option. Do not assume every unfinished book reflects a reading difficulty or lack of effort. Listen to the child’s description before deciding what support is relevant.
Offer a few ways forward
They can return the book, bookmark it for later, read a short part together, or choose another title. Keep the options genuine rather than using them to steer inevitably toward finishing. If the book belongs to a series, the child does not have to complete the whole sequence before deciding it is not their preference. For assigned work, help them describe the obstacle to the teacher and follow the agreed approach rather than quietly substituting a different book.
Keep the next choice inviting
Ask what they might like more of next time: shorter chapters, clearer illustrations, a different subject, or less suspense. Use that information as a starting point, not a permanent label. Avoid maintaining a public list of abandoned books or making a child earn the right to try another one. Optional reading can include sampling. The useful outcome is understanding what the child wants from the next experience and helping them find a workable way into it.