Yes. I think it's a good bet that at least one paladin will be riding Sunny (maybe one rides, and one gets TK'd?). But yeah, this seems like the means by which the paladins get themselves up and into the battle.
From the art, it looks like Calder could have tail smashed Sunny pretty much no matter where they were. On the ground made sense, since Minrah wanted to be with Sunny (maybe helping out, healing, etc), and Haley didn't want to keep holding her up in the air. Of course, no one quite accounted for the bouncability of Sunny. But then, who does?
I am a bit surprised that a single tail slap did that much damage though. Sunny was completely unharmed prior to that hit. I'd expect even a young eye-thingie to have enough HPs to take one hit without going unconscious like that. Then again, rule of cool, and "bouncing beholder" kinda maybe says otherwise...
I'd be shocked if this dungeon isn't hundreds of feet down under the ground through solid rock. Smashing a wall is one thing. Tunneling through a moutainside is a completely different one. It's kind of the quandary that Calder is in. Even if he wins/escapes, where does he go? We don't actually know where the exit to this place is.
They're both highly capable melee combatants. Calder, being a dragon, has tons of HPs, and decreasing those HPs is about the only way to defeat him. Having two more people with strong melee attacks would seem to be incredibly useful in this situation. Paladins get the same to-hit tables as fighters, so even if they don't have weapons on the same level as Roy's, they'll still be a significant asset to fighting against a dragon.
Paladins do get protection from evil as a first level spell. And it does seem to be a pretty obvious one for a paladin to have ready. Paladins also get good saves, and get bonuses to saves (ok, maybe O'chul might not get much of this bonus). If anything, they are the least likely to actually be dominated or affected by spells of any kind of just about anyone in the party.