Au where Elliot didn't kill Robbie he grabbed the astral pulse and snuck out in the night holding Robert. Something in his head told him this would hurt him more knowing his son loves him more then he ever did.
Absolute perfection, taking the most important parts of the Mecha Man legacy- the power source and the future of it- is more of a punishment than killing Robbie.
I feel like Robbie, Mr. Father Of The Year, would notice the astral pulse is missing first and not even think of Robert. He doesn't realize Robert's gone until he's looking for the pulse and he notices how empty the house is all of a sudden.
He makes a missing person report because that's what you're SUPPOSED to do, but he considers finding the pulse way more important than finding his son. Besides, he already suspects who took them, it's just a matter of finding the suspect to get them back.
Meanwhile, this opens up the door for Robert to be raised as a villain! Elliot gives Robert the childhood he never had with Robbie's training getting in the way, and Robert chooses to help Elliot when he gets older, rather than be part of the family legacy he was born into and had no choice in.
