First, diplomacy is hard to level up, in case you finish with less than perfect education. Second, state diplomacy gives very little bonus to opinion about you. Imho it's better to have good traits (diligent+just+brave+kind) than to have high diplomacy stat. Therefore, I usually choose Struggle(martial) or Thrift(business) as childhood focus. Business is easy to level up with Rulership or Business, and Martial... well it's even easier but riskyYou can end up craven maimed incapable for next 20 years.
I'd recommend to read Education_(Conclave) and Focus wiki articles if you want to fully understand what are you doing with education.
Bit of a necro post here, but what is your opinion on intrigue education and using intrigue as a focus?