child Optimus is so little... so small...
What's his relationship with Megatron in the au?
The answer is a bit complicated, especially since I'm currently writing a big story for this AU (which should start releasing soon), but at the beginning, their relationship is built on mutual resentment and emotional distance.
One of the major differences in this AU is that Cybertronians don't and have never had any sparklings. Because of that, the Decepticons, and Megatron in particular, are completely shocked when the AllSpark turns Optimus into a child. They, of course, don't understand his strange behavior that's typical for children, and Megatron is especially short-tempered and frustrated around him.
Over time, however, Megatron finds himself unconsciously seeking out the little one's presence, perhaps even becoming a little dependent on it. Optimus becomes a source of emotions Megatron hasn't experienced in ages: gentle happiness, peace, affection, and even trust. Like a father, he grows fiercely protective of Optimus and takes pride in every one of his accomplishments. After all, Optimus is so small, and both Autobots and Decepticons could hurt him so easily, but the little one is also clever and so willing to grow and learn new things each day.
At the same time, Megatron's pride and reputation prevent him from openly returning Optimus's affection. Instead, he hides behind a gruff attitude and sharp remarks, born from habit alone.
As for Optimus, he's initially terrified of and angry at the Decepticons. Due to the AllSpark's shenanigans, he has no actual memories of his past life, but his frame still carries faint emotional echoes of fear and unease associated with the Decepticons. Megatron's cold, hostile demeanor and his… sadistic tendencies certainly don't help their relationship develop in the early stages.
But with enough time and a looot of patience from both sides, Megatron gradually transforms in Optimus's optics from a terrifying, evil giant into a mentor and role model. Optimus is awestruck by Megatron's leadership abilities, confidence, and intelligence. He desperately wants Megatron's approval and constantly tries to prove his worth, even if it means pushing himself too far or getting hurt in the process. To him, that's still preferable to disappointing Megatron and being left behind, abandoned and forgotten in the unknown world.
So, in the end, they become something like "a smart, grumpy, overworked, lonely father tries to be a caretaker of a smart, overly ambitious, deeply lonely child, burdened with plenty of insecurities."
Both of them are terrified of losing the other. Megatron constantly worries that Optimus will eventually return to the Autobots, and he's already lost too many comrades throughout the Great War to easily trust that someone will stay. Optimus, meanwhile, fears that he'll never truly be enough and that Megatron might one day cast him aside.
In reality ofc, both of them severely underestimate the other's loyalty and capacity for love for the each other.