Ditto on the advice against derringers. They aren't any smaller than a small .380 or .38 (and bigger than a .32), they are very slow to get going, very slow shot-to-shot, you are stuck with a capacity of only two rounds, and reloading takes a long time.
IWB carry is entirely safe as long as the gun is in the holster. The part that requires the most care is actually holstering the gun--if your finger (or part of the holster, or maybe a big shirt button) gets in the trigger guard, you could conceivably pull the trigger and shoot yourself in an indelicate place.
I personally carry an S&W 3913LS 9mm, usually at 1:00 IWB. This gun has a manual safety, so when reholstering, the safety is ALWAYS on, and my thumb is on the exposed hammer to make sure it
stays down. That way even if I were to somehow screw up in a big way, leaving the gun off safe and reholstering with an object in the trigger guard, I could feel the hammer coming back to warn me "er, you might not want to do that." I flip the safety off as soon as it's holstered, since the 3913 is designed to be safety carried hammer down, safety off (redundant internal safeties, firing pin lock, etc.).
Holster is a Galco floppy suede IWB with a plastic retaining clip; I don't recall the model number. I usually insert and remove the gun/holster combo as a unit for safety and convenience reasons, with safety on and thumb on the hammer.
A revolver could also be safety reholstered with your thumb on the hammer, though you don't have the added benefit of a manual safety.
If you're not comfortable with a Kel-Tec or another safety-less gun for IWB carry, you might consider a Beretta Tomcat (.32); very small, not as effective as a .38, but better than a .38 derringer (accurate, fast into action, much higher capacity) and it has a manual safety and an exposed hammer. Just practice enough to ensure you can shoot very accurately. If you can put up with a wider gun, numerous S&W's and Sig 9mm's and .40's are available that are thin enough to carry IWB and have manual safeties.
I am wearing 32/30 jeans right now, but if I were carrying the 3913 at the moment, it'd be a little uncomfortable and I'd need to go to 33/30 or 34/30. If you can fit in a 32, you're not into the "middle age spread" domain yet. A smaller gun would be no problem, though.
I bought a NAA .22 magnum "mini-revolver" because of its small size.
I kept it for about one month.
From five feet away (feet, not yards) I could not get that beast to consistenly print on a large pizza box.
It's one of the very few guns that I've actually sold off after aquiring it.
hillbilly, I had a similar experience. I had an NAA .22LR mini-revolver that I purchased for snakes (with .22 shotshells) when we lived in the boonies, and occasionally carried it out and about (loaded with CCI Stingers). The snake thing didn't pan out; the tiny shotshells didn't have enough oomph to kill even a small snake, so I decided that I wouldn't trust it against the 6' eastern diamondback that had previously been spotted on our property. And it was slow to shoot for defensive purposes, and I wasn't too sure about .22LR out of a 1 1/8" barrel as a defensive load either. I still admired the gun for its quality and workmanship, but decided that for me it was basically an expensive toy, and eventually sold it.