There really should be a main game for such an AU... or just a game in general that allows people to play that out. Fate/Extra has an almost perfect presentation of it.

... Twelve-year-old stupidcool. Interesting. Well! He might see her jumping more than a few double digits of feet up in the air or carrying stuff that looks like she wouldn't be capable of in her small frame... and that she can move a lot faster than normal humans should be capable of doing. Servants are the best combatants - Kimblee should joing the club! Or fight it.