So The Lion Guard Timeline is something to discuss, specifically Kion. According to Ford Riley, TLG takes place after Kiara meets Kovu, but before her first hunt, so let's delve into the entire timeline.

At the end of The Lion King, we see the presentation of Simba and Nala's newborn cub.

In The Lion King II: Simba's Pride, we see the presentation of Simba and Nala's newborn daughter Kiara.

At the beginning of The Lion King II, Kiara meets Kovu, the son of Zira, before being separated by their parents

A few months after Kiara meets Kovu, Kion, the son of Simba and Nala is born.

Kion grows up into a young cub, and meets and befriends Bunga, a honey badger, the adoptive nephew of Timon and Pumbaa

In The Lion Guard: Return of the Roar, Kion gains the power of the Roar of the Elders, and becomes the leader of The Lion Guard, tasked with protecting the Pride Lands, and defending the Circle of Life

Sometime after Return of the Roar, Kion continues his role as leader of the Lion Guard.

Now flash foward 3 years later. Kion is a young adult and continues his role as leader of the Liob Guard. Kion battled the spirt of his Great Uncle Scar, and defeats him once and for all, but in the process is injured by a snake bite. Kion and the Guard set off to find the Tree of Life in order to find the cure.

Kion was born after Kiara met Kovu, and left Pride Rock to find the Tree of Life before her first hunt.