Symbolism, Imagery, Allegory
This play, unfortunately, is full of dead babies and slain children. And it's hard to make jokes about that, even if they are fictional and several hundred years old. The witches throw into their cauldron a "finger of birth-strangled babe" and then conjure an apparition of a bloody child that says Macbeth will not be harmed by any man "of woman born" (4.1.2); Fleance witnesses his father's murder before nearly being killed himself; Macbeth kills Young Siward; and Macduff's young son, his "pretty chicken," is called an "egg" before he's murdered.
So, what's the deal?
The play is fixated on what happens when family lines are extinguished, which is exactly what Macbeth has in mind when he orders the murders of his enemies' children. (His willingness to kill kids, by the way, is a clear sign that he's passed the point of no return.) We can trace all of this back to Macbeth's anger that Banquo's "children shall be kings" (1.3.5), but not Macbeth's: he laments that, when the witches predicted he would be king, they placed a "fruitless crown" upon his head and a "barren scepter" in his hands (3.1.8).
There's also a sense of major political and lineal disorder here. When Macbeth kills Duncan and takes the crown, Malcolm (King Duncan's heir) is denied "the due of birth" (3.6.1). By the play's end, order is restored with the promise of Malcolm being crowned as rightful king. And, we also know that Banquo's line will rule for generations to come. It's fitting that, in the end, Macbeth is killed by a man who was "untimely ripped" from his mother's womb.