Denis Villeneuve Says ‘Dune 3’ Is ‘Not Like a Trilogy’ and Will Be His Last ‘Dune’ Movie: Other Directors Could Take Over So ‘I’m Not Closing the Door’ on the Franchise
I think it depends on the director. Children of Dune is a really good book. It's much better than Dune Messiah IMO which feels more like a short story that wasn't fully sussed out. A well budgeted Children of Dune movie could be very good if they stick to the source material.
Dune Messiah is Herbert's knee-jerk reaction to the reception of Dune (it'll be interesting to see how far Villeneuve goes with this) and is handy jumping-off point for the director. With a bit of a time jump, Children of Dune is also a good place to bring someone else on-board. If the studios are smart they'll get another visionary director in and perhaps keep Villeneuve around as a producer, so he gets to help pick his successor and keep the films on track.