AHHH!!
So many.
I'm obsessed with Ryuu-chan and Acchan but if we're going with OC children than here are a few things:
-Atsushi is a girl dad. If he has a daughter he is dressing up with her and spoiling her
-The kids have Atsushi's appetite but Akutagawa's stand-offish appearance (they're sweet as pie though but have a sharp tongue)
-They're spoiled (from Akutagawa's credit card) but they're well mannered unless given a reason not to be
-They're hyper (Atsushi's stamina)
-They crash hard (Akutagawa's sleepiness)
-They bully skk because they've heard too many stories
-Kyouka and Gin are the only ones trusted (and capable) of watching and taming them