Usage of commas before names of people/titles

I am wondering whether it's necessary to use commas in the sentences below when addressing or introducing someone:

1. “You look after each other, okay children?"

“You look after each other, okay, children?

Which version would be better or are both fine depending on whether there is a pause in speech?

2. "I'll have a sparkling water, please, sir."

Should there be a comma before sir (or does it make the sentence stilted?) or should it be:

"I'll have a sparkling water, please sir."

3. "I turn to see my friend James Grand walking towards me."

Or should it be:

"I turn to see my friend, James Grand, walking towards me."

In the same vein:

"He and his elder sister Evie once looked after us..."

Or should it be:

"He and his elder sister, Evie, once looked after us..."

4. A similar question is regarding commas and the introduction of a name with the possessive:

"We are attending our friends', John and Crystal's, engagement party."

Or should it be:

"We are attending our friends John and Crystal's engagement party."

Any advice on these points would be greatly appreciated!

Let's start with your second-to-last example. Here are the three options you present

I turn to see my friend- James Grand- walking towards me.

I turn to see my friend, James Grand, walking towards me.

I turn to see my friend James Grand walking towards me.

Regarding the first option,

Hyphens are not appropriate in between words except when joining the parts of hyphenated words. Hyphens are used to separate syllables before line breaks. However, dashes (em-dashes) are intended to go in between words. There is a tiny possibility that one might use (em) dashes, as in option one, but only if one wanted, in effect, to "shout" the name, and only as a substitute for the commas of option two (see below). Look up the usage of the hyphen versus the en-dash versus the em-dash.

• http://m.sussex.ac.uk/informatics/punctuation/hyphenanddash/hyphen
• http://m.sussex.ac.uk/informatics/punctuation/hyphenanddash/dash

However, instead of dashes you are far more likely to want commas, or, more likely, no punctuation at all.

Now it may be that you want to markedly pause, for effect, before intoning the name James Grand—as if he were Donald J. Trump or somebody. More likely, though, you are not so ostentatious, and you just want to say which one of your friends appears.

Ostentatiousness aside, whether to use commas comes down to whether you are defining the antecedent (my friend) or simply qualifying it.

If James is my only friend, then I needn't tell you his name to say, which friend. Saying his name doesn't define for you which of my friends he is. This is non-defining information. So there is a comma before the name.

More likely, you will understand I have multiple friends while this one is named James Grand. I am defining which one of my friends this is. There is no comma before the name.

The comma in option two indicates that what follows the comma is nonessential information. It would be nonessential if I have only one friend in the universe. I need not identify or define him by name. I name him, but not in order to identify him. The comma says, you don't need to know his name in order to know which of my friends he is, but I'm telling you.

A better example would be

I saw my husband. (Assume I have only one.)
I saw my husband, James. (I am telling you his name, but the name doesn't change who he is. He's my husband.)
I saw my husband James. (OK, with no comma, the guy's name serves as defining or identifying information. You can assume I have at least one other husband, who is not named James.)

(Maybe I should have illustrated this with horses instead of husbands.)

Perhaps you remember learning about defining versus non-defining relative clauses. The former take no comma, while the latter require a comma.

In archaic English I might say,

Meet my friend, who is James Grand.

See there's a relative pronoun there which. So that's a non-defining relative clause there, after the antecedent. This means I have no friend besides good old James G.

Also in archaic English I could say,

Meet my friend who is James Grand.

Here I have a defining relative clause. I have some friends; this one is James. Meet him.

Take away the who is from the two examples above, and you can still see the defining versus non-defining principle at work.

See the explanation below from http://m.grammarbook.com/punctuation-rules/commas.aspx

Rule 12. If something or someone is sufficiently identified, the description that follows is considered nonessential and should be surrounded by commas.

Examples: Freddy, who has a limp, was in an auto accident.

If we already know which Freddy is meant, the description is not essential.

The boy who has a limp was in an auto accident.

We do not know which boy is meant without further description; therefore, no commas are used.

This leads to a persistent problem. Look at the following sentence:

Example: My brother Bill is here.

Now, see how adding two commas changes that sentence's meaning:

Example: My brother, Bill, is here.

Careful writers and readers understand that the first sentence means I have more than one brother. The commas in the second sentence mean that Bill is my only brother. Why? In the first sentence, Bill is essential information: it identifies which of my two (or more) brothers I'm speaking of. This is why no commas enclose Bill. In the second sentence, Bill is nonessential information—whom else but Bill could I mean?—hence the commas.
Comma misuse is nothing to take lightly. It can lead to a train wreck like this:

Example: Mark Twain's book, Tom Sawyer, is a delight.

Because of the commas, that sentence states that Twain wrote only one book. In fact, he wrote more than two dozen of them.

So I say your option three, with no comma before the name, is correct.