ironically (or maybe with the gender-stereotype breaking intention of my brain, haha?), it's harder for me to picture fem!adashu doing that compared to the canon mxm counterpart. the two aunties are just far too broken to responsibly handle childcare (especially if it's a boy, omg, the poor kid)

canon yu and adachi - especially some late 20s yu, late 30s adachi - wouldn't mind taking care of a child, i believe. but i'd like to think of this only as something that just "kind of happened" instead of them intentionally filling out all the paperwork and actively seeking an adoption. my reason for that specific scenario is, of course, adachi, who barely ever freaking knows what exactly he wants in life and massively sucks at discerning his feelings or being honest with them. he'd insist that he'd hate something and wouldn't ever put in the admittedly huge amount of effort needed for the process.

so, something akin to the 'phoenix becoming trucy's dad' scenario in terms of suddenness maybe? or, better even, the whole 'buddy daddies' shtick? like one day yosuke storms their house with a 5yo kid in tow and rants that the kid had gotten his (yosuke's) address from the mom, who said "here's where your dad lives". the kid repeatedly calls yosuke 'dad', yosuke's eye twitches every time, and he begs yu to take care of the kid ("i can't have the kid at my house! my gf would kill me if she knew!!") while he looks for the mom.. and does the paternity tests, just in case.
adachi isn't a huge fan of the idea at first, for multiple reasons (including - you guessed it - self-deprecation, bc "what good would living with an ex-convict do them?!"). but "it's just for the time being" and yu looks sort of into it, and it's kind of like the old times with nanako... he veery slooowly starts to accept the current state of the things too; the "it's too freaking noisy!" to "silence now makes me uncomfortable" pipeline.
the kid's mom ends up being too busy having fun abroad, paternity tests show no relation to yosuke, but the kid now has nowhere to go. and after some dramatic collective deliberations, it's adachi who sighs and willingly agrees to "for the time being" becoming "sorta, kinda permanent, i guess". the end.

thanks for the question, anon! it's been a while since i wrote about mxm childcare but - just as last time - it can be surprisingly fun once you start thinking about this kind of stuff in detail :>