If you want to be honest about it, try introducing it subtly into conversation. Start with: "You know those fetish threads on (whatever forum, site), I always want to know why someone would become that way".
Then proceed to: "Diaper fetish seems to be a popular one. I dunno, I guess it's just nice to wear something comfy, or it relives stress" etc.
So if they do find out, it won't be as much of a shock.