Not an x reader oneshot, but my last three braincells thought that this would be a pretty cool scenario. Hope you enjoy!
Ludociel looked around the castle of Liones. He thought it looked quite nice for a human structure. However, he didn't come with the other three Archangels to see the castle, he came to see what, or rather, who, was inside of the castle. Rumors had spread far and wide that Meliodas and Elizabeth had a son. It was Ludociel's job, along with the other three Archangels, to see if such things were actually true. It turns out, they were. When the four Archangels walked in, they felt an enormous amount of magical power fly right past them right before they saw the Holy Knight Gilthunder struggling to catch whatever it was that flew past. The Flash Grace's curiosity peaked as he took off after the strange magical aura, followed closely by the other three Archangels who were just as curious. The boy looked panicked and then took off once more, running as fast as his little legs could carry him, before he was trapped in a perfect cube by Merlin. The boy was about to protest to such an annoyance to his escape when he was stopped by none other than the person that the Archangels had come to see: Meliodas. The boy was a little shorter than the demon and looked almost exactly like him. When he spoke, the boy addressed the blonde demon by what the four goddesses were hoping for. "Father, Merlin trapped me in a perfect cube again, that's not fair." he said. Mael slightly snickered, catching the demon's attention as the blonde turned to Ludociel and the other three. He smiled and waved before he spoke to the boy again. "Tristan, you remember how Gowther gave you those memories of the Holy War, Right?" he asked. The four Graces stared in shock that Gowther would do such a thing. The boy nodded and stayed silent. "Well," Meliodas turned and signaled to the child to turn around, "look behind you." Tristan turned around and stared in shock at the four goddesses behind him before he tried to run away again, this time however, he failed as he ran straight into his mother, Elizabeth, before hiding behind her and burying his face into her back. She was taken by surprise by this before she smiled at the young boy and patted his head. "Tristan, these people aren't bad guys, they're nice people." she said. Tristan shook his head and then peeked out from behind his mother. When he saw the Archangels looking at him, he hid behind his mother once more. However, Ludociel quickly flew behind Elizabeth before he picked the young boy up. "Well hey there." he said with a smile. The young boy then finally understood that his parents spoke the truth before he waved back. Ludociel then set the boy back down and then introduced him to the other three Archangels. Tristan's eyes lit up when Ludociel introduced him to Mael. The boy quickly asked if he really could control the sun like in his father's stories, with which the Archangel replied yes he could. When he asked to see him do it, he then summoned a small sun to where the young prince could hold it. When Tristan held it, his eyes lit up and he then thanked him. After a while of talking, the Archangels headed back to the goddess realm to report to the Supreme Deity that she indeed did have a grandson.