According to the Disney Blog:
Cost is $27.99, $13.99 ages 3 to 9. Hours are 5:30-9 p.m. nightly. For reservations, visit Disneyland.com/dine or call 714-781-DINE.
It also depends on whether you planning on going for breakfast (Surf's Up with Mickey and Friends) or dinner. Both are buffet, but breakfast features characters and dinner does not.
wdwinfo.com has the following prices (but no date to indicate how new/old they are):
Character breakfast: Adults: $33.00, Children (3-9) $17.00
Dinner: Adults $27.99, Children (ages 3-9) $13.99.
I would call Disneyland Dining and see if they can give you a more accurate/current pricing.
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