I would suspect those characters may appear in visions (as Anya Taylor-Joy briefly did in DUNE: PART TWO) to set-up a fourth film, and I suspect that for a couple of reasons:
1. The ending of PART TWO, which was pointedly altered from the ending of the novel, had Paul and Chani broken-up. To skip over their reconciliation would render that creative choice completely pointless.
2. Chalamet and Zendaya don't look nearly old enough to convincingly be the parents of teens who look like that. Makeup can only go so far.