There aren't many candidates in this scene.
Macbeth has no children. In Act I, scene 7 Lady Macbeth says,
I have given suck, and know
How tender 'tis to love the babe that milks me
And Macbeth also mentions elsewhere that he has murdered Duncan but Banquo's descendants may become the beneficiaries of his taking that risk.
However, Macbeth has not been mentioned in this scene for around 40 lines, when Ross said, "For that I saw the tyrant's power a-foot". (Macbeth is often referred to as a tyrant in the second half of the play.)
It seems more plausible that the words "He has no children" are spoken as an aside and refer to Malcolm, who has not married yet and therefore has no (legitimate) children. The words would be motivated by Malcolm's rather unsympathetic reaction to the news of Macduff's loss. Malcolm seems more interested in spurring Macduff into revenge (two lines) than in comforting him for his loss (two words):
Let's make us medicines of our great revenge,
To cure this deadly grief.