My brother paid for my two sons (5 & 6) to do it and I believe it was about $130 for the first time. You get offered discounts by email after your first visit.
They have various packages online, but you can also check for vouchers at Costco. If they like it, consider having them go to the kids club, it's one of the better deals
For the first time you have to register them and it will cost more then if you keep bying tickets, but expect it to be more then $60
You can go to the website or call and they tell you or show you
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