I read this on Ao3 in 2019, but it can be a lot older. It was longish and complete. I remember a lot of details but for some reason can’t find it again so it’s possibly undertagged. Anyway: Damian has pissed off a magician of some sort and all of Bruces children get cursed and de-aged. B and Superman have to go to HongKong to get Cass. Something happens to get their memories back and kid-Jason shoots the Joker, Tim was involved somehow. When they turn back, Babs helps Jason get away from Bruce.
You my dear Rijomu are lucky, because I have read this fic and I can tell you exactly where to find it:
The story is Frozen History from Aingeal98:
Bruce Wayne is used to one of his family or co-workers occasionally getting de-aged. What he doesn’t expect is all five of his children reverted to the age of ten. Keeping them all safe is difficult, but he’s Batman, so he’ll handle it.
(He lasts less than twenty-four hours before calling Barbara for help)
Which meets all of your given plot-points except that it isn’t finished yet. The last published chapter however, does feel a lot like an end.
I really hope this is the one! :D