So let's start with the singular 'brother-in-law', which is perfectly clear. If you have a single brother-in-law and he possesses something, this is written as:
My brother-in-law's cooking skills are excellent.
If you have more than one brother-in-law (no possession) you would write:
My brothers-in-law are all brunettes.
This is because when pluralizing a compound noun, we always add the 's' to the most 'important' word. The fact that they are brothers is most important, so it gets the 's'. This is the same for "mothers-in-law", "fathers-in-law", etc.
If you have more than one brother-in-law and they all own something:
My brothers-in-law's restaurant is the best in town!
Confirmation of this final construction can be found at grammarbook.com:
Rule 7
If the compound noun is plural, form the plural first and then use the apostrophe.
Example:
my two brothers-in-law's hats
In addition to the two rules (depending on natural pronunciation) of adding an apostrophe to a noun ending with the letter 's' here on OxforDictionaries, the apostrophe is not added to a noun that already has an apostrophe.
Having said this,
Wendy's product manager insisted...
will serve the purpose and is grammatical.
A couple of examples -
McDonald's chief executive Don Thompson defined under performing Australian and Japanese markets as weak and said the Illinois-based company was concentrating on 'stabilising' key priority markets in both Australia and Japan, as well as Germany and the U.S., reported the Sydney Morning Herald - from DailyMail.
The McDonald's parking lot is full of pickup trucks with fishing gear jutting from their beds, sleek hybrids with kayaks or mountain bikes racked to their roofs or tailgates and family vans packed with kids coming from soccer tournaments - from Tribune.
And finally,
In that same interview, Wendy's chief marketing officer, Craig Bahner, noted that Wendy's customers widely request the pretzel bun for other products. Sure, it costs the customers 30 cents more, he said, but "we totally accommodate. We want them to have the product they want." - from USA Today
A Google search for 'Wendy's chief marketing officer' gives the result with the answer Craig S. Bahner.
A little note: I think when we talk about some authorities from some company, not putting an apostrophe is okay. In other words, when we talk about the designation, apostrophe is not mandatory to use. For instance, "Microsoft CEO said that..."; "IBM spokesperson added that..."; "McDonald's Marketing Manager agreed that...", and an article with the headline and an image caption from NYDailyNews which reads Former Microsoft CEO Steve Ballmer etc.
Best Answer
No. The rule is simple:
s, the possessive suffix is'.'s.Children is an irregular plural, not a regular one. Therefore, the suffix is
's, and the possessive form is children's. Your form *children's' is incorrect.However, the other possessive forms are fine. Parents is a regular plural, so the possessive suffix is simply
'. That means parents' is correct. And you correctly write children's later in the same sentence. (Was your earlier mistake a typo?)In this answer, the * symbol means that the spelling is considered incorrect.