I hope Kairi isn't as pathetic as she was in KH3. I just don't think she has enough experience in wielding the Keyblade like Riku and Sora do.
Which is understandable, considering she used one once
I actually think they'll both pass the exam, but if someone has to fail, I hope it's Sora. I mean, if Riku fails it'll probably be due to darkness, which not only would make sense at this point (Riku doesn't have Darkness left to conquer--he can control it now), but would a repeat of Birth by Sleep. If Sora lost, it wouldn't be what's generally expected, and would be much more interesting. It'd draw parallels to Sora's and Riku's childhood, where Riku always seemed to be "better" than Sora, and generally won their competitions. But at the same time, it'd be new and refreshing, since seeing "the dark guy failing and the light one winning" would be a rehash.
I actually think Riku deserves it more. Riku had to literally work to get his Keyblade. He was supposed to get it and eventually stole it, but after he lost the Kingdom Key back to Sora, he had to live with darkness, learn to control it, and strengthen his heart to the point where he could wield his own Keyblade. Sora got it easy--his heart filled with "friend power" was a get-out-of-jail-free-card for all of his troubles, even when he didn't seem to have the Keyblade ore his friends.
With all of that said, though, I expect them both to pass. Perhaps Sora and Riku fight, and Sora loses yet still passes, or perhaps something happens and Sora doesn't quite pass at first, but then he redeems himself shortly thereafter, or something. Maybe that's not even necessary, and they both pass right off the bat. We know they are both more than likely to become Keyblade Masters, it's just a matter of getting to that point that I'm unsure about. No matter what, I hope Riku isn't the one that fails.
... I suppose the scenario that happens would be dependent on when the MoM exam is taken. If Sora's and Riku's MoM exams occur both at the end, they'd probably both pass. If the MoM exams both occur early on or midway into the storyline, I would expect one to initially fail and then pass by the end of the game. If their MoM exams occur at different points in time, I would expect the one that takes it first to fail (hopefully Sora), the one that takes it second to pass (hopefully Riku), and the first one (Sora) would later re-take the exam and pass.