I'm also of the opinion that some small consumable gift would be a nice thing to do if not required by etiquette. That said, I'd also ask DD's Dad ahead of the trip if he thought that going out to dinner would be reasonable because you want to offer it, making it clear that you'd be comfortable with the three of them going without you if that would be better (assuming you'd be okay with that as an offer, of course). That takes away the risk of offending but still lets you offer if you wish it.
Virg