"I am not so childish."
Translation:No soy tan infantil.
No, "childish" describes someone's personality. You could say "I don't act childish", but that would be a different translation. I have found that it's not extremely helpful to say that estar is temporary and ser is permanent. This is mostly because then you get into debates with yourself about what can or can't be changed. Instead, I teach my students that estar is LOCO: location and condition. It's easier to understand whether something is a condition than whether it's temporary. So, "él está mal" means that he's feeling badly, his condition; and "él es mal" means that he's a bad person - it's part of his personality, who he is. Hope that helps somewhat! The difference between ser and estar is difficult for us English-speakers.
Because you are on the internet, you don't have to buy a dictionary. http://www.wordreference.com/es/translation.asp?tranword=so