Potent cure elixers I believe work on children, you just have to toss it at someone.
However, first you need to get permission. Using potent cure elixirs on a life state is banned and disqualifies you from HOF status if you don't ask first! Please be careful about the advice you give because if Frozen had just gone ahead and used a potent cure elixir on your advice. You would be the reason he lost HOF status/failed because he never actually got permission from Metro or myself.
Story Progression works to make sure that all the life states you want are in the game so you guys shouldn't have to resort to curing a child just to age them up. While I understand wanting a very specific someone, I'd rather not just keep giving permission for you guys to cure the townies when it's banned. There are at least 40 people in a town at one time. More than one is bound to be a werewolf/fairy/witch/vampire as long as they are in your population settings. If you didn't put them in your population settings, than that was part of your strategy and not a game glitch.