My mum read to me uncensored, then she bought me a bazillion books and let me at em, plus ALL the adult books in her rather large bookshelves. By the time I was 7 I knew a lot of very correct and exact latin words. By the time I was 9 I spoke fluent 18th and 19th century English.
All through school other kids all looked at me funny, and refused to talk to me until I'd hit 15 and I'd learned not to say anything... just reflect back.
There may have been some other things going wrong there. Still. It might be wise to tell your 5 year old that the other kids in Grade one may only understand contemporary English. Say, words that only appear in the cannon of the past decade.
Edited, Jan 23rd 2013 2:49pm by Aripyanfar