Would there be any way to modify the game to make the kids age? Like, take existing files from The Sims 3 and put them in Medieval? I tried to do this, but I'm not very experienced...

Grim's medieval core has an aging option for children. It can get a bit wonky and buggy at times (it once forced my monarch to die so her son could take up the job), but that was also around it's first release of that feature. Plenty of bugs still, not as many though and still mostly playable with cc and other mods.
And wow, the community is still active years later.