The first 3 books are really just exposition for God Emperor, and God Emperor is probably impossible to turn into a movie. The majority of the story is just the god emperor talking to people in his bedroom. So I don’t think any film maker could have much ambition of adapting Dune and actually telling the whole story.

The other problem is that Paul is a complete failure. He topples the emperor, but doesn’t have the courage to do what he knows he actually needs to do. His son picks up the mantle after Paul turns into a desert wandering vagrant. So ending at Children would probably also be a bit confusing for a film audience.