The last time Kanna revealed herself to someone her same age level of maturity was when that person was actively being kidnapped. (And also anime-original, iirc, so not directly written by Cool-sensei.)
Actually, no. Chloe learning about Kanna being a dragon is 100% canon. Personally, I think they're just worried that she wouldn't be able to keep her mouth shut about how amazing her best friend is.