Black Manta wouldn't kill a newborn.
For the story to work, I would expect the kid to be atleast 4-5 (or older) and having developped a personality, and then killed off-screen. THAT would be emotional!
As I said, I very much doubt this will happen in the second Aquaman film. He's not a family man yet
Black Manta absolutely IS the type to kill a child, even a newborn.
A lot of fans expect no less because that's a huge part of the power of the character in the Aquaman mythos, hell, generally among DC villains full stop.
Until say, Joker killing Jason Todd, this was the most successful a super villain had been in hurting his nemesis, and one due to the Superfriends show, however one feels about it, that had a fairly high media profile. And it was an event that had really long lasting affects on the character and his world. And all because yes, at the end of the day Black Manta is one of the most heartless, cruel and unrelenting super villains in the DC universe. I get wanting to soften him but... He's a murderous child killer in my book. That's what I want to see in live action.
As for the killed off screen bit...
That is like the opposite of emotional.
"Hey audience... We had this gigantic event happen in the life of our main character you all loved from the last film. No... You don't get to see it. No, you don't even get to experience the interactions and meaning brought to Aquaman's life by having a marriage and family."
I can't see in a typical super hero film this working.