Kris, Aaron, and Zoe Prime were not always the children of Optimus Prime. In fact they're human and were adopted by him. It all happened while he was on the field looking for the source of an energon signal. Of course, as per most times, the Decepticons were their looking for the signal. As the Prime fought, he heard some screams, that sounded like children, coming from somewhere on the field. He had just cleared the place when he heard it so Optimus went to check it out immediately. When Optimus got there, it was a sad sight to see. In fact, it almost made him go out in rage.
A vehicon stood their with his blaster in aim of three children, Aaron, Kris, and Zoe. Behind them were two, bloody, bodies that were most likely dead from the vehicon shooting them. Aaron stood in front of his sisters, acting as a shield, while Zoe was hugging Kris protectively. You can say Optimus Prime straight up kicked the vehicon's aft.
Optimus then gentley introduced himself to the three. They in turn introduced themselves. He asked who were the two people behind them. "Our parents." is what they answered sadly. They had nowhere else to go. They were orphans. The Prime felt pity for them. He had to take them to base anyway.
After some time though, Optimus found out they were techno-organic and decided to adopt them since the three siblings had nowhere to go. Now, they were his little trio of trouble.
