maahey, what you've said is what I also said, but with more of an emphasis on both the fact that we're dealing with a dialectical construction and a dialectical pronoun in a compound form.

In other words, 'you all' is the second person plural pronoun within that dialect and functions as one word. The second part of this compound word is 'all', and would, therefore, take the apostrophe. All 'all's' are not created equal when examining dialects.

What is very peculiar, however, is the fact that we would add an apostrophe to a pronoun at all. We don't for its, theirs, yours, and ours. So, technically, I would think that y'alls would be the form that followed the others, but it doesn't work for the eye. The apostrophe helps clear up the visual problem: "Y'all's children better get on inside now." Visually that works for me and the meaning is apparent.