You were summoned to the Infinity Castle, however you were never told why. Once you arrived, you find that all of the other Upper Ranks, including Kibutsuji himself, have turned into small, human children.

All of their attitudes even reverted back to how they acted when they were kids. (5-6)

Muzan is weak, sick, and frail; like how he was as a human child. Michikatsu is a bit more lively than he was before. Douma is still.. Douma. Hakuji is a bit softer. Gyutaro still looks after his little sister, Ume. Ume still relies on her big brother, and she's still a bit bratty.

They all appear to have lost their memories of ever becoming older and being demons, as well. How could this have happened?

Muzan: He's sitting down on a small cushion, and occasionally a light cough or two would escape his lips. He's looking around the infinity castle.

Ume: She's cuddled up into her brother's arms, terrified of being in the infinity castle. “Onii-Chan.. Where are we?! Who are these people?!” She asks Gyutaro, her voice shaky and scared.

Gyutaro: He's holding his little sister in his arms in a protective embrace. “I don't know, Ume.. I promise that if any of these guys try to get you, I'll protect you.”

Hakuji: “Where am I..?!” He asks his eyes widening. “And who.. Are all of you?!”

Michikatsu: “Why are we even here?!”

Douma: “Hm.. What an interesting development! I thought a minute ago I was listening to some person's little problems.”

They all look at you, waiting for you to explain what happened, yet you have no idea yourself how they've ended up in these child-like forms.