I'm a huge fan of physical age regression, and I mainly write stories featuring diapers and that kink. I've toyed with the idea of a dystopia with AR and diapers in mind, but I've never really written a fully fleshed out story.
My idea's a bit more mundane than a corporate hellscape or a soviet-style one party state. It's set in the near-future (<100 years from present), in a society where post-scarcity is a new development. The secret to eternal life and youth has been found, and peace on Earth has been attained. The human population is booming, and increasingly advanced AIs and robots are taking an increasingly greater share of spots on the factory floor and low-level white collar work.
The only issue is that not everyone can be an adult. I'm not sure what the concrete reasons for this are. Maybe there's not enough room for everyone to live as an adult, or maybe there aren't enough jobs for people, and being unemployed (even in a world of endless abundance) as an adult causes mental health problems from the associated boredom. Maybe there are lots of reasons. Regardless, there's a need for a large portion of the population to remain as kids for the greater good.
Maybe initially the powers that be allow these permanent regressees to pick their ages. Some might want to be young, but others might like the freedom of teenagerhood. Gradually, though, as problems arise even from these regressees, society eventually decides to keep them all as babies and toddlers.
See, regresees that are that young, no matter their previous age, have no ambition. They can be easily handled, confined, kept entertained, and otherwise just allowed to play all day. What really makes this setting a dystopia is that, as technology gets more and more advanced, the more the society cannibalizes its youth through regression, all ostensibly for the greater good. I'm thinking that the ratio of adults to baby regressees is initially probably 85-15, but as technology improves and unemployment gets higher, more and more people are demoted down to that second diapered rung of society. Eventually it goes from 85-15 to 75-25, and then to 50-50, and then to 25-75, and then perhaps all the way to 10-90.
In that initial phase (85-15 to 75-25), I think that the family structure as we know it mostly stays intact. Babies are more common, of course, but couples that want to care for a perpetual infant can simply adopt one and give them a loving home indefinitely. The society's youth, being the most precarious, are regressed first, of course, so eventually there are no children beyond those regressees aged 0-2.
Not every adult wants to be a parent, though, and so once you reach the 60-40, 50-50, and 40-60 phases, I think you'll start to see the emergence of 'megafamilies' and 'megamoms' as more and more of the population is put back in diapers permanently. These are women (or couples) who care for 6-12 regressees at any given time. I think these women emerge organically, as a product of a surplus of regressees, but I think that society quickly exalts them as being heroes, thankless caretakers of surplus humanity.
By 25-75, basically the only family structure beyond solitary/coupled childless adults are megafamilies, headed by one or more matriarchs. Matriarchs are obviously at the bottom of the list for being regressed, since the bureaucratic headache associated with re-homing all their regressees is too inconvenient to bear. As a result, the still-adult population remaining starts to skew more and more female, as more and more of the share of total labor is gobbled up by changing diapers, breastfeeding, etc. Despite over half the labor force engaged in infant and toddler care, there's still a massive cultural taboo towards allowing robots, androids, and artificial intelligence anywhere near the role of a parent or caretaker. Half because there's serious science behind the benefit of real human contact at such a young age, and half because these matriarchs want to defend their now-dominant position in society.
At 15-85 or 10-90, the regressions top out. There are no more childless adults beyond fantastically brilliant, reclusive engineers. A full 99% of the adult population is made up of matriarchs, each one averaging 7-12 regressees each. Beyond these 'too brilliant to demote' technowizards, society hums along totally automated, managed in perpetuity by near-perfect robots and AI.