Definitely check out the Kids Fly Free through Alaska Air. However, because it's one free child's fare per adult flying, you may not find it to be such a great deal. There are other options to save money, of course:
1. We usually book through Alaska Air, as they often have cheap airfares from YVR-LAX (sometimes ~$280 US when on sale).
2. Go through Seattle. You will find the flights to be cheaper from Seatac to LAX or SNA. You can rent a minivan (cost us about $50 for the day thru Budget) to drive down. This way, you don't have to pay daily parking costs at SEATAC. Then rent a minivan for the ride back to Vancouver. (Just make sure you book ahead of time as there are limited cars on their lot that can go across the border.)
3. I book my hotel through Priceline.com. A 3-star hotel (usually the Crowne Plaza, Hyatt Orange County, Anaheim Marriott, Anaheim Hilton, and Anabella Hotel) usually accepts $40-$65 bids. Go to biddingfortravel.com to help you with the Priceline bidding. I have gotten a bid at the Hyatt once for $40/night

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4. If you collect airmiles, you can use them to buy your Disney passes.
5. Check out Jubilee Travel's best bets. They sometimes have package deals (air, transfers, and hotel) for 4 days for around $580/person.
Personally, I like buying my airfare and then using Priceline for hotel. On previous visits, that has always been the best deal for our family.