I saw all three of these at a fitness center.

Problem:
Inconsistent use of a possessive apostrophe makes it meaningless.

Explanation:
“Kid’s Club” says that there is one kid and that he or she has a club. “Kids’ Club” says that there are many kids and that they have a club. “Kids Club” says that there is a club for kids.

Solution:
Know why you’re using a possessive apostrophe.