The ill effects of under 3s in aircraft are usually suffered by the surrounding victims - oops! pax.
I suspect one reason they shriek is ear pain which they can neither understand, vocalise or cure. Immature, delicate ears and kids often being coldy or congested are a poor combination in aircraft, as are shrieking children and fare paying passengers.
A little consideration for both the child and your fellow pax makes the decision to fly, or not, a simple one.