Casually observe a kitteh taking a bath. This strategy requires that you really know your cat so you know how the cat will bathe. Grey kittens, for instance, may bathe for up to an hour. Brown kittens take shorter baths, around ten minutes for a thorough cleaning.
Without blatantly watching the bath, watch for it to near the end. Casually stand up and approach the kitteh. Preferably from behind.
Run your hands through the kittehs fur the wrong way. If you can, stick your fingers between his toes. Kiss him on the head.
As you back slowly away, whisper “you missed a spot.”
Leave the room. Sure, you’ll miss the reaction, but it is the only safe course of action.
I don’t think this would really bother my cats. They must be less goal-oriented than yours.
Oliver likes bathing too much for this to bother him, really. Unless The Boy does it, because The Boy has cooties. But Obi only bathes out of necessity.