If they strike and let go, then it's more of a shock than painful. Even in adults, their teeth are so tiny they only just break the surface of the skin. After a few minutes you won't be able to see a mark, even if they drew a bit of blood.
However it's a bit different if they bite and hang on, which is more of a misplaced feeding response than a "leave me alone" type of bite. Their jaws can exert some force, so it will feel like a hard pinch - enough for you to want to run them under a slow-running cold tap to surprise them into letting go.
Although I may be biased on that second point - I suspect having a Corn hanging off your nose (my only experience) is somewhat more painful than, say, a hand or finger!