It is difficult.
My step children were similar, and they didn't really remember their parents being together either (dss was just 3, and dsd 18 months when their parents split; dss has no real memories of that time - everything he can 'remember' is from home movies, photos etc)
they are now adults, just finishing up at university. dss still hankers after his parents getting back together, nearly 20 years on. he knows it won't happen (even if dh and I split up), but it is the thing that would make him happiest in the world.
It is purely because, I think, it is a fantasy - anything could be true if that were to happen, iyswim? not just his parents being back together, but as you say - perfection. happy families, no grumpy days, sun shining all the time, etc. it is part of what we all feel when we sometimes wish things were a bit different.
I don't know how to handle it - I don't think it was handled particularly well with dh and his ex - for his adult children to still wish for that is a bit extreme, imo.
how does your dss' mum deal with htings (are you on speaking terms?) I know that dh's ex blamed dh all the time, and created false realities there too - eg would repeatedly tell the children that she could not afford xyz becasue 'daddy does not pay enough' (when in fact, he did pay enough, and she had the money to pay for it). and told them again and again they had a difficult life because their parents were divorced (there were things which were hard, yes, but overall, they had a privileged upbringing - nice house, in a nice area, private schools, several foreign holidays a year, etc). I think this contributed hugely to their feelings of wanting their parents back together - because then it would be different, iyswim? probably mostly because their mum would (in their eyes) be a bit happier, maybe.