User blog comment:The Dragon Demands/The History of House Targaryen. Part IV: The Dance of the Dragons/@comment-5014364-20131202203600

After Luke died, Daemon hired two assassins known to history as "Blood and Cheese" to kill one of Helaena's sons, as an "eye for an eye, son for a son" principle. They forced her to pick which of her sons should be killed - she chose Maelor given that he was only 3 and wouldn't understand. They then tormented Helaena by killing the other son instead, torturing Jaehaerys to death.

The implication, due to unreliable narrative sources, is that Helaena may have actually committed suicide out of grief, but everyone blamed Rhaenyra -- possibly that she even intentionally framed her suicide on Rhaenyra's arrival (knowing how much it would taint the Blacks' victory).

Afterwards young Maelor was sent via ship to Oldtown, while Jaehaera fled the city. The Blacks didn't want to kill Jaehaerya nor did many of the betrayer factions because they all know that marrying Aegon II's daughter would cement any claim to the throne.

Yes, Syrax also died during the riot.