Many Characters can Defeat Kaidou who isn't in his Prime yet
When Someone believes that only some Child of Prophecy can Defeat them, it means They are referring to when They are at their Prime,
Kaidou isn't Stupid to believe No One can Defeat at any Moment in his Life, but He believed that when He finally reaches his Full Potential (Prime), then No One can Defeat him
Xebec, Roger, WB & Oden are proven to not have Fought Kaidou in his Prime
And his Meeting with Shanks was Quick suggesting that They didn't actually Fight, but Shanks proved that He has Roger's Level of Haki & Skills
So those Five represent Fighters whom Kaidou met & He believes They were/are Joy Boy Candidates.
As for who Defeated his Non-Prime Version among them? Probably Xebec & WB + We've seen how He almost Died against Oden, and it's 50-50 whether He lost against Roger or not, He didn't lose against Shanks though