Using phonetic type dialogue is very iffy in my opinion. Using the differing expressions and word orders is one thing, but I’d suggest not writing out an accent phonetically. Maybe one or two words that aren’t obtrusive to emphasize the accent but that is best done with a VERY light touch.
By the way, using contractions in dialogue is not confined to YA in my opinion. Unless it’s a part of the characterization (a very formal person for instance) it’s a natural part of most people’s speech.