Massive SPOILERS for House of the Dragon season 2, episode 1, and later events from Fire & Blood, the show's source material.
Has House Of The Dragon Cut Maelor, The 3rd Child Of Aegon & Helaena? Maelor Is Notably Absent From Blood & Cheese, Which Could Change The Overarching Narrative Close In Fire & Blood, Aegon II and Helaena Targaryen have three children: Jaehaerys, Jaehaera, and Maelor. The season 2 premiere prominently featured Jaehaerys as the child who was killed in the Blood & Cheese assassination, but Maelor was notably absent from the episode.
The common folk attempted to decide on Maelor's fate, debating whether they should sell him to Rhaenyra or to the Hightowers. Though it's disputable which way he died, Maelor ended up dying during this debate through some brutal means. This death starts a sequence of violent vengeance from Greens like Ormund Hightower and Daeron Targaryen.
This would, of course, be impossible to pull off without Jaehaera, as she's the only match for Aegon III that would be suitable for the Greens to make the deal. Ultimately, Jaehaera dies by suicide before the marriage can be consummated, ending the bloodline established by Viserys I and Alicent Hightower. But this occurs after the war is over and would likely be after House of the Dragon's ending.