It does show up for the eldest girl. And that's just for us, the audience.
Normally, the reincarnation trope never let's you forget that the mc reincarnated because they'll be using their all knowing knowledge from their past live.
But he just used his past lives knowledge once and that was at the beginning. Maybe the novel has more scenes of the knowledge utilization but that all that I meant
It does hold relevance. In his previous life, he lost all of his family and would come home to an empty house. This is what drove him to take in all those children and become a protective dad in his current life. Him being here with his past memories changed the original events of this world. The story showed glimpses of what happened to the eldest daughter and all the other children in a timeline where Gray wasn’t in their lives, and it was grim. Gray’s slight knowledge of the “characters” in this world also gives him better insight into certain things like the friction between the “protagonist” and the hero that seems predestined but may be overcome with proper support.
I mean, it's just so forgetable of a story point that hardly comes up so it almost feels unnecessary.
so much so that he could have had the same backstory as a resident of that universe and nothing about how he acts would've changed except his reason for helping them out in the beginning was because he knew the kid was the main protag of the story.

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You know, the fact that he reincarnated feels irrelevant to the story because the only instance where his past knowledge was used was; at the very beginning of the story and I think there one more but i can't remember