If you leave out the adjective "wee", consider the verse from Wella Wella (found in the DT:

She had a pen-knife three foot long...

I don't think that many would consider any knife "three foot long" to be "wee" OR what we normally think of as a "pen-knife." I suspect that the author of song didn't either, but it's an exagerration -- you could kill not only a baby, but an adult if you stuck a much shorter blade into their head (as in the song).

Drawing upon my days of teaching hand-to-hand combat in the Army....

The heart lies (mostly) behind the sternum, or breastbone. A blade three inches long, thrust in from the right (your right, as you faced the person) could knick one or more of the left structures (the left of the person you're stabbing, that is). Such a blade would have to be thrust flat, or horizontally, between the ribs (getting the knife blade stuck in the ribs is not desirable). The thrust would also have to be quite forceful to penetrate the cartilage.

A four inch blade would be preferable, but in any case "a pen-knife three foot long" would be overkill.

Another problem would arise in stabbing a woman, and that is penetrating the fatty tissue of the breast far enough to hit the heart. In this case, a horizontal thrust up under the breast and to from right to left would be the most likely to succeed. I think that this could be done with a three or four inch blade.

Do it just right and you might not get much external bleeding, either.