Sweet Mother of God: thank Heavens someone is still willing to take on this topic!
I don’t buy the argument that dialogue won’t sound believably kid-like if it’s actually grammatically correct. Read any book by Natalie Babbitt or E.B. White or Norton Juster, and you’ll be reassured that good grammar wielded well is invisible and takes a back seat to story and character every single time. Whereas I can’t even finish an easy reader in which a talking animal says “I’m taller than her.” No, you’re not!
Head here for the article ‘Apostrophes Don’t Mean, “Here Comes an S.” ‘