Akutagawa is the type who doesn’t know how to baby talk or find the point to, considering his childhood. So I think this is on point I really do this he treats his babies like mini adults. Probably lulls them to sleep reading his paperwork outloud because someone mentioned he has a very soothing, low voice that the kids find comfort in.
Talking in his romance era vampire speak of how, “I shall rip this foe asunder if you want, my little bug, would you like that?” While his kids babble incoherently. Somehow they know exactly what the other is saying.

Akutagawa, holding his baby in his arms, in front of an enemy who he planned to kill but cannot fully decide: what do you think, sweet bat, should we kill him or spare his life?

Baby, bug eyed: buh (Kill him)

Akutagawa, glowing with pride: wonderful choice

Atsushi, in the background: this is why I don’t let you take them to work