I count 8 different "Costco <foobar> worth it" threads currently active.
https://encrypted.google.com/search?oq= ... o+worth+it
The short answer is "we have 5 kids" -> yes. You will save your membership fee in savings on milk and gas alone.
Also you would be surprised on the prices for things like grains and flour. You will probably find Costco's prices lower than your co-op for many of the items you are quoting.