Season 2 use plenty of childhood flashbacks during important point of character development and I kind of love how they're used in vastly different ways.
Edo and Judai's childhood appeared first and it was presented through the lense of their passion for card game and the joy they derived from it. In both case, the contrast between this simpler childhood and their current state comes from a change in how they view duel mosnter. In Edo, it's in their association with his father's death and in Judai it's in the loss of personnal meaning to his duels. In both their case, remembering this childhood is a way of re-connecting with that more innocent joy. Bringing back a bit of that passionate child as a way to grow further.
Then comes Ryo and Asuka in the second part of the season. Their childhood is also portrayed as a time of innocent joy, contrasting their current life. And in both case it's presented as if it could solve things in the same way as it did for Judai and Edo: remind them of their goals and dreams as children so they re-connect with those. The difference is that, in both of their case, Asuka and Ryo find no growth in that childhood. They have changed too much and it just can't be reconciled. Instead they go forward by acknowledging that change and that they're no longer as innocent.
Innocence is a relevant aspect of this. All four of these childhood are defined by Judai, Edo, Ryo and Asuka not knowing as much as their teenage self, not having experienced as such. It's relevant because the final childhood that close the season stands at a complete opposite of that.
The Saiou sibling's childhood appears last and it's neither joyful nor innocent. It's defined by fear and pain but also: knowledge. Saiou and Mizuchi are isolated for their supernatural knowledge. But it's also the experience of the world's violence, and how it denied them a sense of childhood. When Saiou talk about his childhood it's not to re-connect with it, or to accept he has changed. The growth happens in acceptance, re-evaluating it through a kinder lense.
It's all different facet of looking back at childhood. To re-connect, to let go of, to accept.
