Isn't Akatosh technically a Dragon himself?

As Akatosh, he's typically depicted as a Dragon, yes. As Auriel, he's an Eagle or a tall Altmer holding the Sun aloft over his head. As Alkosh he's a big winged cat. His depictions vary by race, but they all stand for the same Divine.

Why did he create the Dragonborn?

Through a process called Mythopoeia, a religious sect in the First Era called the Marukhati Selectives managed to use a powerful artifact to change the nature of Akatosh, making him more pro-man and pro-Mundus. This retroactively rewrote time, and caused Akatosh to actively try to safeguard the current Kalpa or cycle of existence from ending. To that end, he began to send fragments of himself out into the world to help sustain it. The Dragonborn not only can kill Dragons and take their souls, but they can wear the Amulet of Kings and sustain the barrier between Mundus and Oblivion, which is what Alessia and all her heirs did for many centuries.

Aren't the Dragons Akatosh's children?

Yes. They were possibly shards of Akatosh accidentally shed through the tampering of the Selectives. Like Akatosh, they represent immortality and rule through divine might, but unlike him they were cruel.

Alduin first born of Akatosh, why did he bring back the dragonborn to kill his first born son?

Because Alduin became cruel and petty, and because Akatosh wanted to protect Mundus and humanity, due to the tampering of the Selectives which rewrote history. Akatosh would have been very disappointed in his "son", who chose to enslave and kill so many humans. Alduin was originally meant to reset the Kalpa, but Akatosh in his current form would also oppose that, since he's trying to preserve the current Kalpa. The Dragonborn is supposed to kill this corrupted "son" of Akatosh so that the soul can be reabsorbed by Akatosh, extending the lifespan of the world and removing a threat to the mortal races.