Dune has a significant time gap within itself as well.
Children might work as a feature but Messiah is short and strange and very depressing. As a novel, it was divisive. As a film, it would probably be dysfunctional. The filmmakers would either have to gin up some action in order to hold the audience's attention (bad) or rework Messiah and Children as a single, three-act story (less bad).
Television is the other alternative.
Dune has time gap yes, but only two years or so between parts 2 and 3.
But Messiah has enough content it is extremely difficult to combine with much longer Children of Dune. This actually makes it difficult to adapt Children as well... it is so long. A lot can be cut or shortened for film to be sure, but i figure it would feel rushed in one film easily, unless it is a very long film.
While i like Messiah quite a bit, i do feel it could've done with some extra. Adding some expository scenes and action (namely scenes from Paul's Jihad) might not be a bad idea, if done faithfully to Dune's style.
Scifi Channel's Children of Dune series adapted Messiah pretty well in one episode, with the two latter episodes devoted to Children of Dune.
