I may just be repeating what others have said, I didn't read the other responses, sorry.
We are a family of five. Three boys, including a teen son. Much different than your family, LOL.
When my kids are having friends over, I feel good knowing that we have a stocked freezer they can pick from. No, it's not super healthy, but it keeps the gang fed and lessens my food bill.
Some items last for quite a while. Granola bars, Sun Chips, etc... are great because of the price and so easy to grab for lunches, hikes, camping, etc....
Freezer items are fine for quite a while. I buy the bag of boneless, skinless chicken breasts and it's wonderful to pull out what I need and use and zip back up for the next time.
You get fantastic deals on berries at Costco. The berries can be easily freezed.
Toilet paper, dishwasher detergent, laundry soap, cat litter, paper plates and utensils are all great deals and last for quite some time.
Cereal is my big exception. I get better deals at the grocery store with sales and coupons. Since cereal is almost always on sale it's better at the store.
Also, if you have a cell phone carrier who has a kiosk at Costco there are great deals. My husband was ready for an upgrade and we went to the local cell store. The phone was $199. Yes, we would get a $50 rebate, but still.
At Costco, the phone (exact same one) was $80 plus we got a free headset and carrying case.
You get a good discount on gas, but we don't live near Costco. We have to drive out of our way for gas, so I usually buy that at Fred Meyer. However, when I'm at Costco I fill up.
Electronics are a good buy as you have a year on the return. You can't beat that. I rarely return things at Costco, but the few times I have, they have always been polite.
There is also a magazine rack with 30% off the cover price. I treat myself once in a while.
Go take a look around, you can go with a friend or just ask for a day pass to see for yourself.
Hope that helps.