Native speakers are often depressingly illiterate when it comes to apostrophes -- they are often inserted where not needed, or inserted in the wrong place, or omitted where they are necessary.
nickpeake, you thought right. The rule with the possessive apostrophe is this:
If the word is singular, add 's.
If the word is plural and the plural ends in s, simply add an apostrophe.
If the word is plural and the plural does not end in s, add 's.
The following are all correct:
The man's hat. (The hat of the man -- singular, so add 's)
The ladies' books. (The books of the ladies -- plural ending in s, so add an apostrophe)
The women's shoes. (The shoes of the women -- plural not ending in s, so add 's)
A children's book means "A book for children". "Children" is a plural not ending in s, so you must add 's.